COMP 170 L2 L18: Random Variables: Independence and Variance Page 1.

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COMP 170 L2

L18: Random Variables: Independence and VariancePage 1

COMP 170 L2

Outline

Independence of RVs

Distribution Functions of RVs

Independence between RVs

Expectation of product of independent RVs

Variance of RV

Definition and Examples

Additivity

Standard deviation

Central limit theorem

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P(X=k) viewed as a function of k: The distribution function of X.

In general

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Probability Weight and Distribution FunctionPage 4

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Distribution Functions of RVsPage 5

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Distribution Functions of RVsPage 6

For coin example

D(1, 9) = P(1 <= X <=9 ) ~= 1

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Outline

Independence of RVs

Variance of RV

Additivity

Standard deviation

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Independence Between Events

E is independent of F

If P(E|F) = P(E)

The information that “Event F occurred” does not change the

probability of E

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Random Variable and Event

Given a rand variable, we can define many difference events

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Independence between RVsPage 10

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No need to further check

P(X=0, Y=1) ?= P(X=0)P(Y=1)

P(X=1, Y=0) ? = P(X=1)P(Y=0)

P(X=1, Y=1) ?= P(X=1)P(Y=1)

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Draw two cards from a deck of 52 cards

X: number on first card

Y: number on second card

X and Y are independent when drawing with replacement

X and Y are not independent when drawing without replacement

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Outline

Independence of RVs

Variance of RV

Additivity

Standard deviation

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Independence and ExpectationPage 15

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Independence and ExpectationPage 16

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Illustrating Proof via ExamplePage 19

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Illustrating Proof via ExamplePage 20

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Outline

Independence of RVs

Variance of RV

Additivity

Standard deviation

Sum of independent RVs

The Trend

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Variance of RVs

Probability starts with a process whose outcome is uncertain

Sample space: the set of all possible outcomes

A random variable (RV) is a function defined on the sample space

Different runs of the process might yield different outcomes

The RV might take different values in different runs

In other words, the value of RV varies across different runs

Some RVs vary more and some vary less

Number of heads in 1 coin flip

Number of heads in 10 coin flips

Number of heads in 100 coin flips

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Variance of RVs

Variance of an RV X

Measures how much it varies (across different runs of process)

Relative to the mean value

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Which RV vary the most?

Flip fair coin

Number of heads in 1 flip

Number of heads in 10 flips

Number of heads in 100 flips

Variance

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Outline

Independence of RVs

Variance of RV

Additivity

Standard deviation

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Calculating VariancePage 28

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Example 1: We have already seen

Number of heads in n flips

(x1+X2+…+X10)

(X1+X2+…+X100)

Variance

10/4 = 10 * 1/4

100/4 = 100 * 1/4

Additivity is true here.

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Additivity is true here.

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Example 3 (Counter Example) Page 31

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Example 3 (Counter Example) Page 32

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Two LemmasPage 34

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An Application of Theorem 5.29

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A CorollaryPage 38

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Outline

Independence of RVs

Variance of RV

Additivity

Standard deviation

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Standard Deviation

Standard deviation is another measure of how much a rv varies or

how much a distribution spread out

It is the square root of variance.

Highlights the differences between variance and standard

deviation

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Distributions, Variances, and Standard Deviations Page 41

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The examples on the previous page show

Standard deviation is a natural measure of “spread” of distribution

Variance is easier to manipulate mathematically. Will see this in

further study of probability theory and statistics.

Distributions, Variances, and Standard Deviations

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Outline

Independence of RVs

Variance of RV

Additivity

Standard deviation

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A Pattern

If we flip of a coin a large number of times,

“The number of heads” has bell-shaped distribution.

This phenomenon is not unique to coin flipping

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Test with .8 probability of getting correct answer

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Normal DistributionPage 48

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Recap: 13-05-2010

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Recap: 13-05-2010

E is independent of F

If P(E|F) = P(E)

The information that “Event F occurred” does not change the

probability of E

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Recap: 13-05-2010

Flip fair coin

Number of heads in 1 flip: Variance = 1/4

Number of heads in 10 flips: Variance = 10/4

Number of heads in 100 flips: Variance = 100/4

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Number of times until first success

Throw a fair die

How many times, on average, do you need to throw the die until

you see a 1?

P(getting 1 at each throw) = 1/6

Answer: 1/(1/6) = 6

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Throw a fair die

you see a 2 and 3 in that order?

Expected number of throws to see 2: 6

After that, expected number of throws to see 3: 6

Answer: 6+6 = 12

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Throw a fair die

you see a 2 and 3 , where the order does not matter?

P( get 2 or 3 in one throw ) = 1/3

Expected number throws until you see one of 2 or 3: 3

After that, P( get the other number in each throw) = 1/6

Expected number of throws until you see the other number: 6

Answer: 3+6 = 9

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Old Exam Question 1Page 57

P( at least one copy of T1 in 20 weeks)

= 1 – P( no copy of T1 in 20 weeks)

= 1 – P ( no T1 in week1 AND no T1 in week 2 AND …)

= 1- P(no T1 in week1) P(no T1 in week 2) …

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Can we do this?

P(all 10 types of toys in 20 weeks)

= P(copy of T1 in 20 weeks AND copy of T2 in 20 weeks AND …)

= P(copy of T1 in 20 weeks) P(copy of T2 in 20 weeks)…

NO, events not independent

Correct way

P(all 10 types of toys in 20 weeks) = 1 – P(A)

Use inclusion-exclusion to calculate P(A)

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Old Exam Question 2

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COMP 170 L2 L18: Random Variables: Independence and Variance Page 1.

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