Sfs4e ppt 06

Copyright © 2014, 2013, 2010 and 2007 Pearson Education, Inc.

Chapter

Discrete Probability Distributions

6


Section

Discrete Random Variables

6.1


Objectives1. Distinguish between discrete and continuous

random variables

2. Identify discrete probability distributions

3. Construct probability histograms

4. Compute and interpret the mean of a discrete random variable

5. Interpret the mean of a discrete random

6. Compute the standard deviation of a discrete random variable

6-3


Objective 1

• Distinguish between Discrete and Continuous Random Variables

6-4

Copyright © 2014, 2013, 2010 and 2007 Pearson Education, Inc.6-5

A random variable is a numerical measure of the outcome from a probability experiment, so its value is determined by chance. Random variables are denoted using letters such as X.


A discrete random variable has either a finite or countable number of values. The values of a discrete random variable can be plotted on a number line with space between each point.


A continuous random variable has infinitely many values. The values of a continuous random variable can be plotted on a line in an uninterrupted fashion.


Determine whether the following random variables are discrete or continuous. State possible values for the random variable.

(a)The number of light bulbs that burn out in a room of 10 light bulbs in the next year.

(b) The number of leaves on a randomly selected oak tree.

(c) The length of time between calls to 911.

EXAMPLE Distinguishing Between Discrete and Continuous Random Variables

EXAMPLE Distinguishing Between Discrete and Continuous Random Variables

Discrete; x = 0, 1, 2, …, 10

Discrete; x = 0, 1, 2, …

Continuous; t > 0


Objective 2

• Identify Discrete Probability Distributions

5-9


A probability distribution provides the possible values of the random variable X and their corresponding probabilities. A probability distribution can be in the form of a table, graph or mathematical formula.


The table to the right shows the probability distribution for the random variable X, where X represents the number of movies streamed on Netflix each month.

x P(x)

0 0.06

1 0.58

2 0.22

3 0.10

4 0.03

5 0.01

EXAMPLE A Discrete Probability DistributionEXAMPLE A Discrete Probability Distribution


Rules for a Discrete Probability Distribution

Let P(x) denote the probability that the random variable X equals x; then

1. Σ P(x) = 12. 0 ≤ P(x) ≤ 1

Rules for a Discrete Probability Distribution

Let P(x) denote the probability that the random variable X equals x; then

1. Σ P(x) = 12. 0 ≤ P(x) ≤ 1

5-12


EXAMPLE Identifying Probability DistributionsEXAMPLE Identifying Probability Distributions

x P(x)

0 0.16

1 0.18

2 0.22

3 0.10

4 0.30

5 0.01

Is the following a probability distribution?

No. Σ P(x) = 0.97



x P(x)

0 0.16

1 0.18

2 0.22

3 0.10

4 0.30

5 – 0.01


No. P(x = 5) = –0.01



x P(x)

0 0.16

1 0.18

2 0.22

3 0.10

4 0.30

5 0.04


Yes. Σ P(x) = 1


Objective 3

• Construct Probability Histograms

5-16


A probability histogram is a histogram in which the horizontal axis corresponds to the value of the random variable and the vertical axis represents the probability of that value of the random variable.


Draw a probability histogram of the probability distribution to the right, which represents the number of movies streamed on Netflix each month.

EXAMPLE Drawing a Probability HistogramEXAMPLE Drawing a Probability Histogram

x P(x)

0 0.06

1 0.58

2 0.22

3 0.10

4 0.03

5 0.01


Objective 4

• Compute and Interpret the Mean of a Discrete Random Variable

5-19


The Mean of a Discrete Random Variable

The mean of a discrete random variable is given by the formula

where x is the value of the random variable and P(x) is the probability of observing the value x.

The Mean of a Discrete Random Variable

The mean of a discrete random variable is given by the formula

where x is the value of the random variable and P(x) is the probability of observing the value x.

5-20

x x P x


Compute the mean of the probability distribution to the right, which represents the number of DVDs a person rents from a video store during a single visit.

EXAMPLE Computing the Mean of a Discrete Random Variable

EXAMPLE Computing the Mean of a Discrete Random Variable

x P(x)

0 0.06

1 0.58

2 0.22

3 0.10

4 0.03

5 0.01x x P x 0(0.06)1(0.58) 2(0.22) 3(0.10) 4(0.03) 5(0.01)

1.49


Interpretation of the Mean of a Discrete Random VariableSuppose an experiment is repeated n independent times and the value of the random variable X is recorded. As the number of repetitions of the experiment increases, the mean value of the n trials will approach μX, the mean of the random variable X. In other words, let x1 be the value of the random variable X after the first experiment, x2 be the value of the random variable X after the second experiment, and so on. Then

The difference between and μX gets closer to 0 as n increases.

Interpretation of the Mean of a Discrete Random VariableSuppose an experiment is repeated n independent times and the value of the random variable X is recorded. As the number of repetitions of the experiment increases, the mean value of the n trials will approach μX, the mean of the random variable X. In other words, let x1 be the value of the random variable X after the first experiment, x2 be the value of the random variable X after the second experiment, and so on. Then

The difference between and μX gets closer to 0 as n increases.5-22

x

x1 x2 L xn

nx


The following data represent the number of DVDs rented by 100 randomly selected customers in a single visit. Compute the mean number of DVDs rented.


49.1100

... 10021

xxx

X


As the number of trials of the experiment increases, the mean number of rentals approaches the mean of the probability distribution.


Objective 5

• Compute and Interpret the Mean of a Discrete Random Variable

5-26


Because the mean of a random variable represents what we would expect to happen in the long run, it is also called the expected value, E(X), of the random variable.


EXAMPLE Computing the Expected Value of a Discrete Random VariableEXAMPLE Computing the Expected Value of a Discrete Random Variable

A term life insurance policy will pay a beneficiary a certain sum of money upon the death of the policy holder. These policies have premiums that must be paid annually. Suppose a life insurance company sells a $250,000 one year term life insurance policy to a 49-year-old female for $530. According to the National Vital Statistics Report, Vol. 47, No. 28, the probability the female will survive the year is 0.99791. Compute the expected value of this policy to the insurance company.

x P(x)

530 0.99791

530 – 250,000 = -249,470

0.00209

Survives

Does not survive

E(X) = 530(0.99791) + (-249,470)(0.00209)

= $7.50


Objective 6

• Compute the Standard Deviation of a Discrete Random Variable

5-29


Standard Deviation of a Discrete Random Variable

The standard deviation of a discrete random variable X is given by

where x is the value of the random variable, μX is the mean of the random variable, and P(x) is the probability of observing a value of the random variable.

Standard Deviation of a Discrete Random Variable

The standard deviation of a discrete random variable X is given by

where x is the value of the random variable, μX is the mean of the random variable, and P(x) is the probability of observing a value of the random variable.

5-30

X x x 2 P x

x2 P x X2


Compute the variance and standard deviation of the following probability distribution which represents the number of DVDs a person rents from a video store during a single visit. Remember, the mean that we found was 1.49.

EXAMPLE Computing the Variance and Standard Deviation of a Discrete Random VariableEXAMPLE Computing the Variance and Standard Deviation of a Discrete Random Variable

x P(x)

0 0.06

1 0.58

2 0.22

3 0.10

4 0.03

5 0.01


x μ P(x)

0 1.49 –1.49 2.2201 0.06 0.133206

1 1.49 –0.49 0.2401 0.58 0.139258

2 1.49 0.51 0.2601 0.22 0.057222

3 1.49 1.51 2.2801 0.1 0.22801

4 1.49 2.51 6.3001 0.03 0.189003

5 1.49 3.51 12.3201 0.01 0.123201

EXAMPLE Computing the Variance and Standard Deviation of a Discrete Random Variable

EXAMPLE Computing the Variance and Standard Deviation of a Discrete Random Variable

X2 x X 2 P(x)0.8699

x 0.8699

0.9327

x X 2x X x X 2

P(x)


Section

The Binomial Probability Distribution

6.2


Objectives

1. Determine whether a probability experiment is a binomial experiment

2. Compute probabilities of binomial experiments

3. Compute the mean and standard deviation of a binomial random variable

4. Construct binomial probability histograms

5-34


Criteria for a Binomial Probability ExperimentCriteria for a Binomial Probability Experiment

An experiment is said to be a binomial experiment if

1. The experiment is performed a fixed number of times. Each repetition of the experiment is called a trial.

2. The trials are independent. This means the outcome of one trial will not affect the outcome of the other trials.

3. For each trial, there are two mutually exclusive (or disjoint) outcomes, success or failure.

4. The probability of success is fixed for each trial of the experiment.

Criteria for a Binomial Probability ExperimentCriteria for a Binomial Probability Experiment

An experiment is said to be a binomial experiment if

1. The experiment is performed a fixed number of times. Each repetition of the experiment is called a trial.

2. The trials are independent. This means the outcome of one trial will not affect the outcome of the other trials.

3. For each trial, there are two mutually exclusive (or disjoint) outcomes, success or failure.

4. The probability of success is fixed for each trial of the experiment.

5-35


Notation Used in the Binomial Probability Notation Used in the Binomial Probability DistributionDistribution

•There are n independent trials of the experiment.

•Let p denote the probability of success so that 1 – p is the probability of failure.

•Let X be a binomial random variable that denotes the number of successes in n independent trials of the experiment.So, 0 < x < n.

Notation Used in the Binomial Probability Notation Used in the Binomial Probability DistributionDistribution

•There are n independent trials of the experiment.

•Let p denote the probability of success so that 1 – p is the probability of failure.

•Let X be a binomial random variable that denotes the number of successes in n independent trials of the experiment.So, 0 < x < n.

5-36


Which of the following are binomial experiments?

(a) A player rolls a pair of fair die 10 times. The number X of 7’s rolled is recorded.

EXAMPLE Identifying Binomial ExperimentsEXAMPLE Identifying Binomial Experiments

Binomial experiment



(b) The 11 largest airlines had an on-time percentage of 84.7% in November, 2001 according to the Air

Travel Consumer Report. In order to assess reasons for delays, an official with the FAA randomly selects flights until she finds 10 that were not on time. The number of flights X that need to be selected is recorded.


Not a binomial experiment – not a fixed number of trials.



(c) In a class of 30 students, 55% are female. The instructor randomly selects 4 students. The

number X of females selected is recorded.


Not a binomial experiment – the trials are not independent.


Objective 2

• Compute Probabilities of Binomial Experiments

5-40


According to the Air Travel Consumer Report, the 11 largest air carriers had an on-time percentage of 79.0% in May, 2008. Suppose that 4 flights are randomly selected from May, 2008 and the number of on-time flights X is recorded.

Construct a probability distribution for the random variable X using a tree diagram.

EXAMPLE Constructing a Binomial Probability DistributionEXAMPLE Constructing a Binomial Probability Distribution


Binomial Probability Distribution Function

The probability of obtaining x successes in n independent trials of a binomial experiment is given by

where p is the probability of success.

Binomial Probability Distribution Function

The probability of obtaining x successes in n independent trials of a binomial experiment is given by

where p is the probability of success.

5-42

P x n Cx px 1 p n xx 0,1,2,...,n


Phrase Math Symbol

“at least” or “no less than”or “greater than or equal to” ≥

“more than” or “greater than” >

“fewer than” or “less than” <

“no more than” or “at most”or “less than or equal to ≤

“exactly” or “equals” or “is” =


EXAMPLE Using the Binomial Probability Distribution FunctionEXAMPLE Using the Binomial Probability Distribution Function

According to the Experian Automotive, 35% of all car-owning households have three or more cars.

(a)In a random sample of 20 car-owning households, what is the probability that exactly 5 have three or more cars?

P(5) 20 C5 (0.35)5 (1 0.35)20 5

0.1272




(b) In a random sample of 20 car-owning households, what is the probability that less than 4 have three or more cars?

P(X 4) P(X 3)

P(0) P(1) P(2) P(3)

0.0444




(c) In a random sample of 20 car-owning households, what is the probability that at least 4 have three or more cars?

P(X 4) 1 P(X 3)

1 0.0444

0.9556


Objective 3

• Compute the Mean and Standard Deviation of a Binomial Random Variable

5-47


Mean (or Expected Value) and Standard Deviation of a Binomial Random Variable

A binomial experiment with n independent trials and probability of success p has a mean and standard deviation given by the formulas

Mean (or Expected Value) and Standard Deviation of a Binomial Random Variable

A binomial experiment with n independent trials and probability of success p has a mean and standard deviation given by the formulas

5-48

X np and X np 1 p


According to the Experian Automotive, 35% of all car-owning households have three or more cars. In a simple random sample of 400 car-owning households, determine the mean and standard deviation number of car-owning households that will have three or more cars.

EXAMPLE Finding the Mean and Standard Deviation of a Binomial Random Variable

EXAMPLE Finding the Mean and Standard Deviation of a Binomial Random Variable

X np

(400)(0.35)

140

X np(1 p)

(400)(0.35)(1 0.35)

9.54


Objective 4

• Construct Binomial Probability Histograms

5-50


(a) Construct a binomial probability histogram with n = 8 and p = 0.15.

(b) Construct a binomial probability histogram with n = 8 and p = 0. 5.

(c) Construct a binomial probability histogram with n = 8 and p = 0.85.

For each histogram, comment on the shape of the distribution.

EXAMPLE Constructing Binomial Probability Histograms






Construct a binomial probability histogram with n = 25 and p = 0.8. Comment on the shape of the distribution.















For a fixed probability of success, p, as the number of trials n in a binomial experiment increase, the probability distribution of the random variable X becomes bell-shaped.As a general rule of thumb, if np(1 – p) > 10, then the probability distribution will be approximately bell-shaped.

For a fixed probability of success, p, as the number of trials n in a binomial experiment increase, the probability distribution of the random variable X becomes bell-shaped.As a general rule of thumb, if np(1 – p) > 10, then the probability distribution will be approximately bell-shaped.

5-61


Use the Empirical Rule to identify unusual observations in a binomial experiment.

The Empirical Rule states that in a bell-shaped distribution about 95% of all observations lie within two standard deviations of the mean.


The Empirical Rule states that in a bell-shaped distribution about 95% of all observations lie within two standard deviations of the mean.

5-62



95% of the observations lie between μ – 2σ and μ + 2σ.

Any observation that lies outside this interval may be considered unusual because the observation occurs less than 5% of the time.


95% of the observations lie between μ – 2σ and μ + 2σ.

Any observation that lies outside this interval may be considered unusual because the observation occurs less than 5% of the time.

5-63


According to the Experian Automotive, 35% of all car-owning households have three or more cars. A researcher believes this percentage is higher than the percentage reported by Experian Automotive. He conducts a simple random sample of 400 car-owning households and found that 162 had three or more cars. Is this result unusual ?

EXAMPLE Using the Mean, Standard Deviation and Empirical Rule to Check for Unusual Results in a Binomial Experiment


X np

(400)(0.35)

140

X np(1 p)

(400)(0.35)(1 0.35)

9.54




The result is unusual since 162 > 159.1

X np

(400)(0.35)

140

X np(1 p)

(400)(0.35)(1 0.35)

9.54

X 2 X 140 2(9.54)

120.9

X 2 X 140 2(9.54)

159.1

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Sfs4e ppt 06

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