Dr. Ka-fu Wong

Ka-fu Wong © 2003 Chap 6- 1

Dr. Ka-fu Wong

ECON1003Analysis of Economic Data

Ka-fu Wong © 2003 Chap 6- 2l

GOALS

1. Define the terms random variable and probability distribution.

2. Distinguish between a discrete and continuous probability distributions.

3. Calculate the mean, variance, and standard deviation of a discrete probability distribution.

4. Describe the characteristics and compute probabilities using the binomial probability distribution.

5. Describe the characteristics and compute probabilities using the hypergeometric distribution.

6. Describe the characteristics and compute the probabilities using the Poisson distribution.

Chapter SixDiscrete Probability DistributionsDiscrete Probability Distributions


Random Variables

A random variable is a numerical value determined by the outcome of an experiment.

A probability distribution is the listing of all possible outcomes of an experiment and the corresponding probability.


Types of Probability Distributions

A discrete probability distribution can assume only certain outcomes.

A continuous probability distribution can assume an infinite number of values within a given range.



Examples of a discrete distribution are:The number of students in a class.The number of children in a family.The number of cars entering a carwash in

a hour.Number of home mortgages approved by

Coastal Federal Bank last week.Number of CDs you own.Number of t rips made outside Hong

Kong in the past one year.The number of ten-cents coins in your

pocket.



Examples of a continuous distribution include:The distance students travel to class.The time it takes an executive to drive to

work.The length of an afternoon nap.The length of time of a particular phone

call. The amount of money spent on your last

haircut.


Features of a Discrete Distribution

The main features of a discrete probability distribution are:The sum of the probabilities of the

various outcomes is 1.00.The probability of a particular outcome is

between 0 and 1.00.The outcomes are mutually exclusive.


Example 1

Consider a random experiment in which a coin is tossed three times. Let x be the number of heads. Let H represent the outcome of a head and T the outcome of a tail.

The possible outcomes for such an experiment will be:

TTT, TTH, THT, THH, HTT, HTH, HHT, HHH.

Thus the possible values of x (number of heads) are 0,1,2,3


EXAMPLE 1 continued

The outcome of zero heads occurred once. TTT

The outcome of one head occurred three times. TTH, THT, HTT

The outcome of two heads occurred three times. THH, HTH, HHT

The outcome of three heads occurred once. HHH

From the definition of a random variable, x as defined in this experiment, is a random variable.



The Mean of a Discrete Probability Distribution

The mean: reports the central location of the data. is the long-run average value of the

random variable. is also referred to as its expected value,

E(X), in a probability distribution. is a weighted average.


The Mean of a Discrete Probability Distribution

The mean is computed by the formula:

where represents the mean and P(x) is the probability of the various outcomes x.

Σ[xP(x)]μ

Similar to the formula for computing grouped mean where P(x) is replaced by relative frequency.


The Variance of a Discrete Probability Distribution

The variance measures the amount of spread (variation) of a distribution.

The variance of a discrete distribution is denoted by the Greek letter 2 (sigma squared).

The standard deviation is the square root of 2.


The Variance of a Discrete Probability Distribution

The variance of a discrete probability distribution is computed from the formula:

P(x)]μ)Σ[(xσ 22

Similar to the formula for computing grouped variance where P(x) is replaced by relative frequency.


EXAMPLE 2

Dan Desch, owner of College Painters, studied his records for the past 20 weeks and reports the following number of houses painted per week:

# o f H o u s e s P a i n t e d W e e k s

10 5

11 6

12 7

13 2


EXAMPLE 2 continued

Probability Distribution:

Number of houses painted, x W e e k s Probability, P(x)

10 5 .25 11 6 .30

12 7 .35

13 2 .10 Total 20 1.00


EXAMPLE 2 continued

Compute the mean number of houses painted per week:

11.3

(13)(.10)(12)(.35)(11)(.30)(10)(.25)

Σ[xP(x)]E(x)μ


EXAMPLE 2 continued

Compute the variance of the number of houses painted per week:

0.91

0.28900.17150.02700.4225

(.10)11.3)(13...(.25)11.3)(10

P(x)]μ)Σ[(xσ22

22


Binomial Probability Distribution

The binomial distribution has the following characteristics:An outcome of an experiment is

classified into one of two mutually exclusive categories, such as a success or failure.

The data collected are the results of counts.

The probability of success stays the same for each trial.

The trials are independent.



To construct a binomial distribution, let n be the number of trials x be the number of observed

successes be the probability of success on each

trial

The formula for the binomial probability distribution is:

P(x) = nCx x(1- )n-x


The density functions of binomial distributions with n=20 and different success rates p



The formula for the binomial probability distribution is:P(x) = nCx x(1- )n-x


X=number of heads The coin is fair, i.e., P(head) = 1/2.

P(x=0) = 1/8 P(x=1) = 3/8 P(x=2) = 3/8 P(x=3) = 1/8

When the coin is not fair, simple counting rule will not work.


EXAMPLE 3

The Alabama Department of Labor reports that 20% of the workforce in Mobile is unemployed. From a sample of 14 workers, calculate the following probabilities:Exactly three are unemployed.At least three are unemployed.At least one are unemployed.


EXAMPLE 3 continued

The probability of exactly 3:

The probability of at least 3 is:

2501.

)0859)(.0080)(.364(

)20.1()20(.)3( 113314

CP

551.000....172.250.

)80(.)20(....)80(.)20(.)3( 0141414

113314

CCxP

The Alabama Department of Labor reports that 20% of the workforce in Mobile is unemployed. From a sample of 14 workers


Example 3 continued

The probability of at least one being unemployed.

.956.0441

.20)(1(.20)C1

P(0)11)P(x140

014

The Alabama Department of Labor reports that 20% of the workforce in Mobile is unemployed. From a sample of 14 workers


Mean & Variance of the Binomial Distribution

The mean is found by:

The variance is found by:

n

)1(2 n


EXAMPLE 4

From EXAMPLE 3, recall that =.2 and n=14.

Hence, the mean is:= n = 14(.2) = 2.8.

The variance is:2 = n (1- ) = (14)(.2)(.8) =2.24.


Finite Population

A finite population is a population consisting of a fixed number of known individuals, objects, or measurements. Examples include:The number of students in this class.The number of cars in the parking lot.The number of homes built in Blackmoor.


Hypergeometric Distribution

The hypergeometric distribution has the following characteristics:There are only 2 possible outcomes.The probability of a success is not the

same on each trial. It results from a count of the number of

successes in a fixed number of trials.


EXAMPLE 8 of last lecture

R1

B1

R2

B2

R2

B2

7/12

5/12

6/11

5/11

7/11

4/11

In a bag containing 7 red chips and 5 blue chips you select 2 chips one after the other without replacement.

The probability of a success (red chip) is not the same on each trial.



The formula for finding a probability using the hypergeometric distribution is:

where N is the size of the population, S is the number of successes in the population, x is the number of successes in a sample of n observations.

nN

xnSNxS

C

CCxP

))(()(



Use the hypergeometric distribution to find the probability of a specified number of successes or failures if:the sample is selected from a finite

population without replacement (recall that a criteria for the binomial distribution is that the probability of success remains the same from trial to trial)

the size of the sample n is greater than 5% of the size of the population N .


The density functions of hypergeometric distributions with N=100, n=20 and different success rates p (=S/N).


EXAMPLE 5

The National Air Safety Board has a list of 10 reported safety violations. Suppose only 4 of the reported violations are actual violations and the Safety Board will only be able to investigate five of the violations. What is the probability that three of five violations randomly selected to be investigated are actually violations?

238.252

)15(4))((

))(()3(

510

2634

510

2541034

C

CC

C

CCP


Poisson Probability Distribution

The binomial distribution becomes more skewed to the right (positive) as the probability of success become smaller.

The limiting form of the binomial distribution where the probability of success is small and n is large is called the Poisson probability distribution.

The formula for the binomial probability distribution is:P(x) = nCx x(1- )n-x



The Poisson distribution can be described mathematically using the formula:

where is the mean number of successes in a particular interval of time, e is the constant 2.71828, and x is the number of successes.

!)(

x

exP

x



The mean number of successes can be determined in binomial situations by n , where n is the number of trials and the probability of a success.

The variance of the Poisson distribution is also equal to n .

X, the number of success generally has no specific upper limit.

Probability distribution always skewed to the right. Becomes symmetrical when gets

large.


EXAMPLE 6

The Sylvania Urgent Care facility specializes in caring for minor injuries, colds, and flu. For the evening hours of 6-10 PM the mean number of arrivals is 4.0 per hour. What is the probability of 2 arrivals in an hour?

1465.!2

4

!)(

42

e

x

exP

x


- END -

Chapter SixDiscrete Probability DistributionsDiscrete Probability Distributions

Dr. Ka-fu Wong

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