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Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2 Binomial Probability Distribution 6.3 Continuous Random Variables and the Normal Probability Distribution 6.4 Standard Normal Distribution 6.5 Applications of the Normal Distribution
Transcript
Page 1: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Chapter 6 Random Variables and the Normal Distribution

61 Discrete Random Variables

62 Binomial Probability Distribution

63 Continuous Random Variables and the Normal Probability Distribution

64 Standard Normal Distribution

65 Applications of the Normal Distribution

61 Discrete Random Variables

Objectives

By the end of this section I will be

able tohellip

1) Identify random variables

2) Explain what a discrete probability distribution is and construct probability distribution tables and graphs

3) Calculate the mean variance and standard deviation of a discrete random variable

Random Variables

A variable whose values are determined by chance

Chance in the definition of a random variable is crucial

Example 62 - Notation for random variables

Suppose our experiment is to toss a single fair

die and we are interested in the number

rolled We define our random variable X to be

the outcome of a single die roll

a Why is the variable X a random variable

b What are the possible values that the

random variable X can take

c What is the notation used for rolling a 5

d Use random variable notation to express the probability of rolling a 5

Example 62 continued

Solution

a) We donrsquot know the value of X before we toss the die which introduces an element of chance into the experiment

b) Possible values for X 1 2 3 4 5 and 6

c) When a 5 is rolled then X equals the outcome 5 or X = 5

d) Probability of rolling a 5 for a fair die is 16 thus P(X = 5) = 16

Types of Random Variables

Discrete random variable - either a finite number of values or countable number of values where ldquocountablerdquo refers to the fact that there might be infinitely many values but they result from a counting process

Continuous random variable infinitely many values and those values can be associated with measurements on a continuous scale without gaps or interruptions

Identify each as a discrete or continuous random variable

(a) Total amount in ounces of soft drinks you consumed in the past year

(b) The number of cans of soft drinks that you consumed in the past year

Example

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 2: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

61 Discrete Random Variables

Objectives

By the end of this section I will be

able tohellip

1) Identify random variables

2) Explain what a discrete probability distribution is and construct probability distribution tables and graphs

3) Calculate the mean variance and standard deviation of a discrete random variable

Random Variables

A variable whose values are determined by chance

Chance in the definition of a random variable is crucial

Example 62 - Notation for random variables

Suppose our experiment is to toss a single fair

die and we are interested in the number

rolled We define our random variable X to be

the outcome of a single die roll

a Why is the variable X a random variable

b What are the possible values that the

random variable X can take

c What is the notation used for rolling a 5

d Use random variable notation to express the probability of rolling a 5

Example 62 continued

Solution

a) We donrsquot know the value of X before we toss the die which introduces an element of chance into the experiment

b) Possible values for X 1 2 3 4 5 and 6

c) When a 5 is rolled then X equals the outcome 5 or X = 5

d) Probability of rolling a 5 for a fair die is 16 thus P(X = 5) = 16

Types of Random Variables

Discrete random variable - either a finite number of values or countable number of values where ldquocountablerdquo refers to the fact that there might be infinitely many values but they result from a counting process

Continuous random variable infinitely many values and those values can be associated with measurements on a continuous scale without gaps or interruptions

Identify each as a discrete or continuous random variable

(a) Total amount in ounces of soft drinks you consumed in the past year

(b) The number of cans of soft drinks that you consumed in the past year

Example

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 3: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Random Variables

A variable whose values are determined by chance

Chance in the definition of a random variable is crucial

Example 62 - Notation for random variables

Suppose our experiment is to toss a single fair

die and we are interested in the number

rolled We define our random variable X to be

the outcome of a single die roll

a Why is the variable X a random variable

b What are the possible values that the

random variable X can take

c What is the notation used for rolling a 5

d Use random variable notation to express the probability of rolling a 5

Example 62 continued

Solution

a) We donrsquot know the value of X before we toss the die which introduces an element of chance into the experiment

b) Possible values for X 1 2 3 4 5 and 6

c) When a 5 is rolled then X equals the outcome 5 or X = 5

d) Probability of rolling a 5 for a fair die is 16 thus P(X = 5) = 16

Types of Random Variables

Discrete random variable - either a finite number of values or countable number of values where ldquocountablerdquo refers to the fact that there might be infinitely many values but they result from a counting process

Continuous random variable infinitely many values and those values can be associated with measurements on a continuous scale without gaps or interruptions

Identify each as a discrete or continuous random variable

(a) Total amount in ounces of soft drinks you consumed in the past year

(b) The number of cans of soft drinks that you consumed in the past year

Example

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 4: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example 62 - Notation for random variables

Suppose our experiment is to toss a single fair

die and we are interested in the number

rolled We define our random variable X to be

the outcome of a single die roll

a Why is the variable X a random variable

b What are the possible values that the

random variable X can take

c What is the notation used for rolling a 5

d Use random variable notation to express the probability of rolling a 5

Example 62 continued

Solution

a) We donrsquot know the value of X before we toss the die which introduces an element of chance into the experiment

b) Possible values for X 1 2 3 4 5 and 6

c) When a 5 is rolled then X equals the outcome 5 or X = 5

d) Probability of rolling a 5 for a fair die is 16 thus P(X = 5) = 16

Types of Random Variables

Discrete random variable - either a finite number of values or countable number of values where ldquocountablerdquo refers to the fact that there might be infinitely many values but they result from a counting process

Continuous random variable infinitely many values and those values can be associated with measurements on a continuous scale without gaps or interruptions

Identify each as a discrete or continuous random variable

(a) Total amount in ounces of soft drinks you consumed in the past year

(b) The number of cans of soft drinks that you consumed in the past year

Example

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 5: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example 62 continued

Solution

a) We donrsquot know the value of X before we toss the die which introduces an element of chance into the experiment

b) Possible values for X 1 2 3 4 5 and 6

c) When a 5 is rolled then X equals the outcome 5 or X = 5

d) Probability of rolling a 5 for a fair die is 16 thus P(X = 5) = 16

Types of Random Variables

Discrete random variable - either a finite number of values or countable number of values where ldquocountablerdquo refers to the fact that there might be infinitely many values but they result from a counting process

Continuous random variable infinitely many values and those values can be associated with measurements on a continuous scale without gaps or interruptions

Identify each as a discrete or continuous random variable

(a) Total amount in ounces of soft drinks you consumed in the past year

(b) The number of cans of soft drinks that you consumed in the past year

Example

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 6: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Types of Random Variables

Discrete random variable - either a finite number of values or countable number of values where ldquocountablerdquo refers to the fact that there might be infinitely many values but they result from a counting process

Continuous random variable infinitely many values and those values can be associated with measurements on a continuous scale without gaps or interruptions

Identify each as a discrete or continuous random variable

(a) Total amount in ounces of soft drinks you consumed in the past year

(b) The number of cans of soft drinks that you consumed in the past year

Example

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 7: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Identify each as a discrete or continuous random variable

(a) Total amount in ounces of soft drinks you consumed in the past year

(b) The number of cans of soft drinks that you consumed in the past year

Example

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 8: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(a) continuous

(b) discrete

Example

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 9: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Identify each as a discrete or continuous random variable

(a) The number of movies currently playing in US theaters

(b) The running time of a randomly selected movie

(c) The cost of making a randomly selected movie

Example

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 10: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(a) discrete

(b) continuous

(c) continuous

Example

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 11: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Discrete Probability Distributions

Provides all the possible values that the random variable can assume

Together with the probability associated with each value

Can take the form of a table graph or formula

Describe populations not samples

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 12: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

The probability distribution table of

the number of heads observed when

tossing a fair coin twice

Table 62 in your textbook

Example

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 13: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Probability Distribution of a Discrete Random Variable

The sum of the probabilities of all the possible values of a discrete random variable must equal 1

That is ΣP(X) = 1

The probability of each value of X must be between 0 and 1 inclusive

That is 0 le P(X ) le 1

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 14: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Let the random variable x represent the number of girls in a family of four children Construct a table describing the probability distribution

Example

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 15: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Determine the outcomes with a tree diagram

Example

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 16: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Total number of outcomes is 16

Total number of ways to have 0 girls is 1

Total number of ways to have 1 girl is 4

Total number of ways to have 2 girls is 6

06250161girls) 0(P

25000164girl) 1(P

3750166girls) 2(P

Example

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 17: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Total number of ways to have 3 girls is 4

Total number of ways to have 4 girls is 1

25000164girls) 3(P

06250161girls) 4(P

Example

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 18: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Distribution is

NOTE

x P(x)

0 00625

1 02500

2 03750

3 02500

4 00625

1)(xP

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 19: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Mean of a Discrete Random Variable

The mean μ of a discrete random variable represents the mean result when the experiment is repeated an indefinitely large number of times

Also called the expected value or expectation of the random variable X

Denoted as E(X )

Holds for discrete and continuous random variables

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 20: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Finding the Mean of a Discrete Random Variable

Multiply each possible value of X by its probability

Add the resulting products

X P X

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 21: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Variability of a Discrete Random Variable Formulas for the Variance and Standard

Deviation of a Discrete Random Variable

22

2

Definition Formulas

X P X

X P X

2 2 2

2 2

Computational Formulas

X P X

X P X

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 22: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

02)(xxP

)(xPx 2x )(2 xPx

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 23: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

x P(x)

0 00625 0 0 0

1 02500 025 1 02500

2 03750 075 4 15000

3 02500 075 9 22500

4 00625 025 16 10000

)(xPx 2x )(2 xPx

000010000400005)( 222 xPx

0100001

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 24: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Discrete Probability Distribution as a Graph Graphs show all the information contained in

probability distribution tables

Identify patterns more quickly

FIGURE 61 Graph of probability distribution for Kristinrsquos financial gain

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 25: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Page 270

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 26: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Probability distribution (table)

Omit graph

x P(x)

0 025

1 035

2 025

3 015

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 27: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Page 270

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 28: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

ANSWER

(a) Probability X is fewer than 3

850

250350250

)2P()1( )0(

)2 1 0(

XXPXP

XXXP

scored goals ofnumber X

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 29: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

ANSWER

(b) The most likely number of goals is the

expected value (or mean) of X

She will most likely score one goal

x P(x)

0 025 0

1 035 035

2 025 050

3 015 045

)(xPx

131)(xxP

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 30: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

ANSWER

(c) Probability X is at least one

750

150250350

)3P()2( )1(

)3 2 1(

XXPXP

XXXP

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 31: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

Section 61 introduces the idea of random variables a crucial concept that we will use to assess the behavior of variable processes for the remainder of the text

Random variables are variables whose value is determined at least partly by chance

Discrete random variables take values that are either finite or countable and may be put in a list

Continuous random variables take an infinite number of possible values represented by an interval on the number line

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 32: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

Discrete random variables can be described using a probability distribution which specifies the probability of observing each value of the random variable

Such a distribution can take the form of a table graph or formula

Probability distributions describe populations not samples

We can find the mean μ standard deviation σ and variance σ2 of a discrete random variable using formulas

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 33: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

62 Binomial Probability Distribution

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 34: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

62 Binomial Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Explain what constitutes a binomial experiment

2) Compute probabilities using the binomial probability formula

3) Find probabilities using the binomial tables

4) Calculate and interpret the mean variance and standard deviation of the binomial random variable

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 35: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Factorial symbol

For any integer n ge 0 the factorial symbol n is defined as follows

0 = 1

1 = 1

n = n(n - 1)(n - 2) middot middot middot 3 middot 2 middot 1

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 36: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Find each of the following

1 4

2 7

Example

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 37: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

Example

504012345677 2

2412344 1

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 38: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Factorial on Calculator

Calculator

7 MATH PRB 4

which is

Enter gives the result 5040

7

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 39: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Combinations

An arrangement of items in which

r items are chosen from n distinct items

repetition of items is not allowed (each item is distinct)

the order of the items is not important

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 40: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

The number of different possible 5 card

poker hands Verify this is a combination

by checking each of the three properties

Identify r and n

Example of a Combination

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 41: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Five cards will be drawn at random from a deck of cards is depicted below

Example

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 42: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

An arrangement of items in which

5 cards are chosen from 52 distinct items

repetition of cards is not allowed (each card is distinct)

the order of the cards is not important

Example

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 43: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Combination Formula

The number of combinations of r items chosen

from n different items is denoted as nCr and

given by the formula

n r

nC

r n r

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 44: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Find the value of

Example

47 C

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 45: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

Example

35

57

123

567

)123()1234(

1234567

34

7

)47(4

747 C

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 46: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Combinations on Calculator

Calculator

7 MATH PRB 3nCr 4

To get

Then Enter gives 35

47 C

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 47: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Determine the number of different

possible 5 card poker hands

Example of a Combination

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 48: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

9605982552C

Example

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 49: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Motivational Example

Genetics bull In mice an allele A for agouti (gray-brown

grizzled fur) is dominant over the allele a which determines a non-agouti color Suppose each parent has the genotype Aa and 4 offspring are produced What is the probability that exactly 3 of these have agouti fur

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 50: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Motivational Example

bull A single offspring has genotypes

Sample Space

A a

A AA Aa

a aA aa

aaaAAaAA

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 51: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Motivational Example

bull Agouti genotype is dominant bull Event that offspring is agouti

bull Therefore for any one birth

aAAaAA

43genotype) agouti(P

41genotype) agoutinot (P

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 52: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Motivational Example

bull Let G represent the event of an agouti offspring and N represent the event of a non-agouti

bull Exactly three agouti offspring may occur in four different ways (in order of birth)

NGGG GNGG GGNG GGGN

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 53: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Motivational Example

bull Consecutive events (birth of a mouse) are independent and using multiplication rule

256

27

4

3

4

3

4

3

4

1

)()()()()( GPGPGPNPGGGNP

256

27

4

3

4

3

4

1

4

3

)()()()()( GPGPNPGPGGNGP

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 54: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Motivational Example

256

27

4

3

4

1

4

3

4

3

)()()()()( GPNPGPGPGNGGP

256

27

4

1

4

3

4

3

4

3

)()()()()( NPGPGPGPNGGGP

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 55: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Motivational Example

bull P(exactly 3 offspring has agouti fur)

4220256

108

4

1

4

34

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

4

3

4

3

4

3

4

3

4

1

N)birth fourth OR Nbirth thirdOR Nbirth second OR Nbirth first (

3

P

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 56: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Experiment

Agouti fur example may be considered a binomial experiment

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 57: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Experiment

Four Requirements

1) Each trial of the experiment has only two possible outcomes (success or failure)

2) Fixed number of trials

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 58: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Experiment

Agouti fur example may be considered a binomial experiment

1) Each trial of the experiment has only two possible outcomes (success=agouti fur or failure=non-agouti fur)

2) Fixed number of trials (4 births)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (frac34)

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 59: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Probability Distribution

When a binomial experiment is performed the set of all of possible numbers of successful outcomes of the experiment together with their associated probabilities makes a binomial probability distribution

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 60: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Probability Distribution Formula

For a binomial experiment the probability of observing exactly X successes in n trials where the probability of success for any one trial is p is

where

XnX

Xn ppCXP )1()(

XnX

nCXn

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 61: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Probability Distribution Formula

Let q=1-p

XnX

Xn qpCXP )(

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 62: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Rationale for the Binomial

Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

The number of outcomes with exactly x successes among n

trials

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 63: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Probability Formula

P(x) = bull px bull qn-x n

(n ndash x )x

Number of outcomes with exactly x successes among n

trials

The probability of x successes among n

trials for any one particular order

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 64: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Agouti Fur Genotype Example

4220256

108

4

1

64

274

4

1

4

34

4

1

4

3

3)34(

4)(

13

13

XP

fur agouti birth with a ofevent X

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 65: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

2ND VARS

Abinompdf(4 75 3)

Enter gives the result 0421875

n p x

Binomial Probability Distribution Formula Calculator

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 66: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Distribution Tables

n is the number of trials

X is the number of successes

p is the probability of observing a success

FIGURE 67 Excerpt from the binomial tables

See Example 616 on page 278 for more information

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 67: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 284

Example

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 68: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

Example

00148

)50()50(15504

)50()50(

)5(

155

5205

520C

XP

heads ofnumber X

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 69: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Binomial Mean Variance and Standard Deviation

Mean (or expected value) μ = n p

Variance

Standard deviation

npqpq 2 then 1 Use

)1(2 pnp

npqpnp )1(

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 70: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

20 coin tosses

The expected number of heads

Variance and standard deviation

Example

10)500)(20(np

05)500)(500)(20(2 npq

2425

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 71: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 284

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 72: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (makes the basket or does not make the basket)

2) Fixed number of trials (50)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 584=0584)

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 73: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(a)

Example

05490

)4160()5840(

)25(

2525

2550C

XP

baskets ofnumber X

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 74: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(b) The expected value of X

The most likely number of baskets is 29

Example

229)5840)(50(np

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 75: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(c) In a random sample of 50 of OrsquoNealrsquos shots he is expected to make 292 of them

Example

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 76: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 285

Example

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 77: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Is this a Binomial Distribution

Four Requirements

1) Each trial of the experiment has only two possible outcomes (contracted AIDS through injected drug use or did not)

2) Fixed number of trials (120)

3) Experimental outcomes are independent of each other

4) Probability of observing a success remains the same from trial to trial (assumed to be 11=011)

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 78: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(a) X=number of white males who contracted AIDS through injected drug use

Example

08160

)890()110(

)10(

11010

10120C

XP

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 79: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(b) At most 3 men is the same as less than or equal to 3 men

Why do probabilities add

Example

)3()2()1()0()3( XPXPXPXPXP

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 80: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Use TI-83+ calculator 2ND VARS to

get Abinompdf(n p X)

Example

= binompdf(120 11 0) + binompdf(120 11 1)

+ binompdf(120 11 2) + binompdf(120 113)

)3()2()1()0( XPXPXPXP

0000554 4E 5553517238

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 81: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(c) Most likely number of white males is the expected value of X

Example

213)110)(120(np

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 82: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(d) In a random sample of 120 white males with AIDS it is expected that approximately 13 of them will have contracted AIDS by injected drug use

Example

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 83: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 286

Example

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 84: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(a)

Example

74811)890)(110)(120(2 npq

43374811

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 85: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

RECALL Outliers and z Scores

Data values are not unusual if

Otherwise they are moderately

unusual or an outlier (see page 131)

2score-2 z

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 86: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Sample Population

Z score Formulas

s

xxz x

z

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 87: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Z-score for 20 white males who contracted AIDS through injected drug use

It would not be unusual to find 20 white males who contracted AIDS through injected drug use in a random sample of 120 white males with AIDS

981433

21320z

Example

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 88: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

The most important discrete distribution is the binomial distribution where there are two possible outcomes each with probability of success p and n independent trials

The probability of observing a particular number of successes can be calculated using the binomial probability distribution formula

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 89: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

Binomial probabilities can also be found using the binomial tables or using technology

There are formulas for finding the mean variance and standard deviation of a binomial random variable

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 90: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

63 Continuous Random Variables and the Normal Probability Distribution

Objectives

By the end of this section I will be

able tohellip

1) Identify a continuous probability distribution

2) Explain the properties of the normal probability distribution

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 91: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

(a) Relatively small sample (n = 100) with large class widths (05 lb)

FIGURE 615- Histograms

(b) Large sample (n = 200) with smaller class widths (02 lb)

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 92: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Figure 615 continued

(c) Very large sample (n = 400) with very small class widths (01 lb)

(d) Eventually theoretical histogram of entire population becomes smooth curve with class widths arbitrarily small

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 93: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Continuous Probability Distributions

A graph that indicates on the horizontal axis the range of values that the continuous random variable X can take

Density curve is drawn above the horizontal axis

Must follow the Requirements for the Probability Distribution of a Continuous Random Variable

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 94: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Requirements for Probability Distribution of a Continuous Random Variable

1) The total area under the density curve

must equal 1 (this is the Law of Total

Probability for Continuous Random

Variables)

2) The vertical height of the density curve can never be negative That is the density curve never goes below the horizontal axis

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 95: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Probability for a Continuous Random Variable

Probability for Continuous Distributions

is represented by area under the curve

above an interval

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 96: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

The Normal Probability Distribution

Most important probability distribution in the world

Population is said to be normally distributed the data values follow a normal probability distribution

population mean is μ

population standard deviation is σ

μ and σ are parameters of the normal distribution

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 97: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

FIGURE 619

The normal distribution is symmetric about its mean μ (bell-shaped)

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 98: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Properties of the Normal Density Curve (Normal Curve)

1) It is symmetric about the mean μ

2) The highest point occurs at X = μ because symmetry implies that the mean equals the median which equals the mode of the distribution

3) It has inflection points at μ-σ and μ+σ

4) The total area under the curve equals 1

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 99: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Properties of the Normal Density Curve (Normal Curve) continued

5) Symmetry also implies that the area under the curve to the left of μ and the area under the curve to the right of μ are both equal to 05 (Figure 619)

6) The normal distribution is defined for values of X extending indefinitely in both the positive and negative directions As X moves farther from the mean the density curve approaches but never quite touches the horizontal axis

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 100: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

The Empirical Rule

For data sets having a distribution that is

approximately bell shaped the following

properties apply

About 68 of all values fall within 1

standard deviation of the mean

About 95 of all values fall within 2

standard deviations of the mean

About 997 of all values fall within 3

standard deviations of the mean

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 101: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

FIGURE 623 The Empirical Rule

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 102: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Drawing a Graph to Solve Normal Probability Problems

1 Draw a ldquogenericrdquo bell-shaped curve with a horizontal number line under it that is labeled as the random variable X

Insert the mean μ in the center of the number line

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 103: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Steps in Drawing a Graph to Help You Solve Normal Probability Problems

2) Mark on the number line the value of X indicated in the problem

Shade in the desired area under the

normal curve This part will depend on what values of X the problem is asking about

3) Proceed to find the desired area or probability using the empirical rule

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 104: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 295

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 105: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

Example

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 106: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 296

Example

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 107: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

Example

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 108: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 296

Example

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 109: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

Example

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 110: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

Continuous random variables assume infinitely many possible values with no gap between the values

Probability for continuous random variables consists of area above an interval on the number line and under the distribution curve

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 111: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

The normal distribution is the most important continuous probability distribution

It is symmetric about its mean μ and has standard deviation σ

One should always sketch a picture of a normal probability problem to help solve it

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 112: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

64 Standard Normal Distribution

Objectives

By the end of this section I will be

able tohellip

1) Find areas under the standard normal curve given a Z-value

2) Find the standard normal Z-value given an area

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 113: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

The Standard Normal (Z) Distribution

A normal distribution with

mean μ = 0 and

standard deviation σ = 1

When the normal curve is plotted we plot Z on the horizontal axis

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 114: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

The Empirical Rule For Standard Normal Distribution

0 -3 -2 -1 1 2 3

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 115: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Find the area that lies under the standard normal curve between Z=-1 and Z=2

Example

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 116: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

034+034+0135=0815

Example

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 117: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Find the area that lies under the standard normal curve that lies above Z=3

Example

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 118: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

Since 100-997=03=0003

we take half of 0003 to get

00015

Example

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 119: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

The Standard Normal (Z) Distribution

In fact we can use a table look-up method to compute the area under the curve for a standard normal distribution given any interval for Z

Table C in Appendix on pages T-9 and T-10

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 120: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

1 It is designed only for the standard normal distribution which has a mean of 0 and a standard deviation of 1

2 It is on two pages with one page for negative z-scores and the other page for positive z-scores

3 Each value in the body of the table is a cumulative area from the left up to a vertical boundary above a specific z-score

Table C Standard Normal Distribution

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 121: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Table continues to -00 in the appendix

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 122: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Table continues to 34 in the appendix

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 123: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

4 When working with a graph avoid confusion

between z-scores and areas

z Score

Distance along horizontal scale of the standard

normal distribution refer to the leftmost

column and top row of Table C

Area

Region under the curve refer to the values in

the body of Table C

5 The part of the z-score denoting hundredths is

found across the top

Table C Standard Normal Distribution

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 124: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Steps for finding areas under the standard normal curve

Table 67

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 125: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 307

Do problems 18 and 20

Example

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 126: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(18)

Example

Use Table C for Z=200

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 127: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

(20)

Example

Use Table C for Z=0500

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 128: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 307

Do problems 26 and 28

Example

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 129: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 130: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

26(c)

Use Table C to find that the area to the left of Z=212 is 09830 We want the area to the right of Z=212 which will be

1-09830=0017

Example

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 131: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 132: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

28(c)

Use Table C to find that the area to the left of Z=-069 is 02451 We want the area to the right of Z=-069 which will be

1-02451=07549

Example

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 133: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 308

Do problems 32 and 36

Example

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 134: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 135: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 308

Do problems 32 and 38

Example

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 136: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 137: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

32(c)

Use Table C to find that the area to the left of Z=128 is 08997

Use Table C to find that the area to the left of Z=196 is 09750

We want the area to the between

Z=128 and Z=196 which will be

09750-08997=00753

Example

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 138: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 139: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

ANSWER

38(c)

Use Table C to find that the area to the left of Z=-201 is 00222

Use Table C to find that the area to the left of Z=237 is 09911

We want the area to the between

Z=-201 and Z=237 which will be

09911-00222=09689

Example

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 140: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 308

Example

Do problem 42

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 141: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 142: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 308

Example

Do problem 52

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 143: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 144: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 308

Example

Do problem 58

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 145: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example

Note this answer is the Z value in the

table that corresponds to

1 - 05120 = 04880

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 146: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

The standard normal distribution has mean μ=0 and standard deviation σ=1

This distribution is often called the Z distribution

The Z table and technology can be used to find areas under the standard normal curve

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 147: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

In the Z table the numbers on the outside are values of Z and the numbers inside are areas to the left of values of Z

The Z table and technology can also be used to find a value of Z given a probability or an area under the curve

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 148: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

65 Applications of the Normal

Distribution

Objectives

By the end of this section I will be

able tohellip

1) Compute probabilities for a given value of any normal random variable

2) Find the appropriate value of any normal random variable given an area or probability

3) Calculate binomial probabilities using the normal approximation to the binomial distribution (OMIT)

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 149: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Standardizing a Normal Random Variable

Any normal random variable X can be transformed into the standard normal random variable Z by standardizing using the formula

xZ

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 150: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Finding Probabilities for Any Normal Distribution Step 1

Determine the random variable X the mean μ and the standard deviation σ

Draw the normal curve for X and shade the desired area

Step 2 Standardize by using the formula Z = (X - μ )σ to find the values of Z corresponding to the X-values

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 151: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Finding Probabilities for Any Normal Distribution Step 3

Draw the standard normal curve and shade the area corresponding to the shaded area in the graph of X

Step4

Find the area under the standard normal curve using either (a) the Z table or (b) technology

This area is equal to the area under the normal curve for X drawn in Step 1

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 152: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 323

Example

Do problems 681012

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 153: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solutions

6) 01587

8) 00062

10) 08413

12) 08400

Example

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 154: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Calculator Use

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 155: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Calculator Use

Do problem 12 again with calculator

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 156: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Calculator Use

Do problems 68 again with calculator

Or use 10^99 as larger value and -10^99

as the smaller value

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 157: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Finding Normal Data Values for Specified Probabilities

Step 1

Determine X μ and σ and draw the normal curve for X

Shade the desired area

Mark the position of X1 the unknown value of X

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 158: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Finding Normal Data Values for Specified Probabilities

Step 2

Find the Z-value corresponding to the desired area

Look up the area you identified in Step 1 on the inside of the Z table

If you do not find the exact value of your area by convention choose the area that is closest

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 159: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Finding Normal Data Values for Specified Probabilities

Step 3

Transform this value of Z into a value of X which is the solution

Use the formula X1 = Zσ + μ

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 160: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 323

Example

Do problems 182022

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 161: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solutions

18) X=572

20) X=5355 and X=8645

22) X=442 and X=958

Example

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 162: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Calculator Use

Do problem 18 and 22 again with calculator

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 163: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Calculator Use

Do problem 18 and 22 again with calculator

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 164: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example 638 - Finding the X-values that mark the Boundaries of the middle 95 of X-values

Edmundscom reported that the average

amount that people were paying for a 2007

Toyota Camry XLE was $23400 Let X = price

and assume that price follows a normal

distribution with μ = $23400 and σ = $1000

Find the prices that separate the middle

95 of 2007 Toyota Camry XLE prices from

the bottom 25 and the top 25

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 165: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example 638 continued

Solution

Step 1

Determine X μ and σ and draw the normal curve for X

Let X = price μ = $23400 and σ = $1000

The middle 95 of prices are delimited by X1 and X2 as shown in Figure 649

FIGURE 649

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 166: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example 638 continued

Solution

Step 2

Find the Z-values corresponding to the desired area

The area to the left of X1 equals 0025 and the area to the left of X2 equals 0975

Looking up area 0025 on the inside of the Z table gives us Z1 = ndash196

Looking up area 0975 on the inside of the Z table gives us Z2 = 196

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 167: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Example 638 continued

Solution

Step 3

Transform using the formula X1 = Zσ + μ

X1 = Z1σ + μ

=(ndash196)(1000) + 23400 = 21440

X2 =Z2σ + μ

=(196)(1000) + 23400 = 25360

The prices that separate the middle 95 of 2007 Toyota Camry XLE prices from the bottom 25 of prices and the top 25 of prices are $21440 and $25360

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 168: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 324

Example

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 169: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solution

Let X be the random variable that represents how many points McGrady scores in a randomly chosen game Find

Example

)30(XP

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 170: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solution

Example

04013

598701

)30(1)30( XPXP

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 171: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 324

Example

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 172: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solution

Example

14640)2010( XP

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 173: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 324

Example

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 174: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solution

Find so that

that is the 5th percentile (only 5 of his games did he have a lower score)

Example

050)( 1XXP

1X

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 175: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solution

Gives the value

Example

050)( 1XXP

points 84141X

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 176: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Page 324

Example

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 177: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Solution

The z-score for X=40 points is 150 Since this z-score is more than -2 and less than 2 this is not unusual

Example

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 178: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

Section 65 showed how to solve normal probability problems for any conceivable normal random variable by first standardizing and then using the methods of Section 64

Methods for finding probabilities for a given value of the normal random variable X were discussed

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ

Page 179: Chapter 6: Random Variables and the Normal Distribution 6 ...jga001/chapter 6 Alford.pdf · Chapter 6: Random Variables and the Normal Distribution 6.1 Discrete Random Variables 6.2

Summary

For any normal probability distribution values of X can be found for given probabilities using the ldquobackwardrdquo methods of Section 64 and the formula X1 = Z1σ + μ


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