Chapter 5 Discrete Probability Distributions - Texas A&M...

Chapter 5Discrete Probability Distributions

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� Random Variables� Discrete Probability Distributions� Expected Value and Variance

A random variable is a numerical description of theoutcome of an experiment.

Random Variables

A discrete random variable may assume either afinite number of values or an infinite sequence ofvalues.

A continuous random variable may assume anynumerical value in an interval or collection ofintervals.

Let x = number of TVs sold at the store in one day,where x can take on 5 values (0, 1, 2, 3, 4)

Example: JSL Appliances

� Discrete random variable with a finite number of values

Let x = number of customers arriving in one day,where x can take on the values 0, 1, 2, . . .

Example: JSL Appliances

� Discrete random variable with an infinite sequence of values

We can count the customers arriving, but there is nofinite upper limit on the number that might arrive.

Random Variables

Question Random Variable x Type

Familysize

x = Number of dependentsreported on tax return

Discrete

Distance fromhome to store

x = Distance in miles fromhome to the store site

Continuous

Own dogor cat

x = 1 if own no pet;= 2 if own dog(s) only; = 3 if own cat(s) only; = 4 if own dog(s) and cat(s)

Discrete

The probability distribution for a random variabledescribes how probabilities are distributed overthe values of the random variable.

We can describe a discrete probability distributionwith a table, graph, or equation.

Discrete Probability Distributions

The probability distribution is defined by aprobability function, denoted by f(x), which providesthe probability for each value of the random variable.

The required conditions for a discrete probabilityfunction are:


f(x) > 0

Σf(x) = 1

� a tabular representation of the probabilitydistribution for TV sales was developed.

� Using past data on TV sales, …

NumberUnits Sold of Days

0 801 502 403 104 20

200

x f(x)0 .401 .252 .203 .054 .10

1.00

80/200


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0 1 2 3 40 1 2 3 4Values of Random Variable x (TV sales)Values of Random Variable x (TV sales)

Prob

abili

tyPr

obab

ility


� Graphical Representation of Probability Distribution

Discrete Uniform Probability Distribution

The discrete uniform probability distribution is thesimplest example of a discrete probabilitydistribution given by a formula.

The discrete uniform probability function is

f(x) = 1/n

where:n = the number of values the random

variable may assume

the values of therandom variableare equally likely

Expected Value and Variance

The expected value, or mean, of a random variableis a measure of its central location.

The variance summarizes the variability in thevalues of a random variable.

The standard deviation, σ, is defined as the positivesquare root of the variance.

Var(x) = σ 2 = Σ(x - μ)2f(x)Var(x) = σ 2 = Σ(x - μ)2f(x)

E(x) = μ = Σxf(x)E(x) = μ = Σxf(x)

� Expected Value

expected number of TVs sold in a day

x f(x) xf(x)0 .40 .001 .25 .252 .20 .403 .05 .154 .10 .40

E(x) = 1.20


� Variance and Standard Deviation

01234

-1.2-0.20.81.82.8

1.440.040.643.247.84

.40

.25

.20

.05

.10

.576

.010

.128

.162

.784

x - μ (x - μ)2 f(x) (x - μ)2f(x)

Variance of daily sales = σ 2 = 1.660

x

TVssquared

Standard deviation of daily sales = 1.2884 TVs


E(x) = 1.20

Binomial Distribution

� Four Properties of a Binomial Experiment

3. The probability of a success, denoted by p, doesnot change from trial to trial.

4. The trials are independent.

2. Two outcomes, success and failure, are possibleon each trial.

1. The experiment consists of a sequence of nidentical trials.

stationarityassumption


Our interest is in the number of successesoccurring in the n trials.

We let x denote the number of successesoccurring in the n trials.

where:f(x) = the probability of x successes in n trials

n = the number of trialsp = the probability of success on any one trial

( )!( ) (1 )!( )!

x n xnf x p px n x

−= −−

( )!( ) (1 )!( )!

x n xnf x p px n x

−= −−


� Binomial Probability Function

( )!( ) (1 )!( )!

x n xnf x p px n x

−= −−

( )!( ) (1 )!( )!

x n xnf x p px n x

−= −−


!!( )!

nx n x−

!!( )!

nx n x−

( )(1 )x n xp p −− ( )(1 )x n xp p −−

� Binomial Probability Function

Probability of a particularsequence of trial outcomeswith x successes in n trials

Number of experimentaloutcomes providing exactly

x successes in n trials


� Example: Evans ElectronicsEvans is concerned about a low retention rate for

employees. In recent years, management has seen a turnover of 10% of the hourly employees annually. Thus, for any hourly employee chosen at random, management estimates a probability of 0.1 that the person will not be with the company next year.


� Using the Binomial Probability FunctionChoosing 3 hourly employees at random, what is

the probability that 1 of them will leave the company this year?

f x nx n x

p px n x( ) !!( )!

( )( )=−

− −1f x nx n x

p px n x( ) !!( )!

( )( )=−

− −1

1 23!(1) (0.1) (0.9) 3(.1)(.81) .2431!(3 1)!

f = = =−

1 23!(1) (0.1) (0.9) 3(.1)(.81) .2431!(3 1)!

f = = =−

Let: p = .10, n = 3, x = 1

� Using Tables of Binomial Probabilities

n x .05 .10 .15 .20 .25 .30 .35 .40 .45 .50

3 0 .8574 .7290 .6141 .5120 .4219 .3430 .2746 .2160 .1664 .12501 .1354 .2430 .3251 .3840 .4219 .4410 .4436 .4320 .4084 .37502 .0071 .0270 .0574 .0960 .1406 .1890 .2389 .2880 .3341 .37503 .0001 .0010 .0034 .0080 .0156 .0270 .0429 .0640 .0911 .1250

pn x .05 .10 .15 .20 .25 .30 .35 .40 .45 .50

3 0 .8574 .7290 .6141 .5120 .4219 .3430 .2746 .2160 .1664 .12501 .1354 .2430 .3251 .3840 .4219 .4410 .4436 .4320 .4084 .37502 .0071 .0270 .0574 .0960 .1406 .1890 .2389 .2880 .3341 .37503 .0001 .0010 .0034 .0080 .0156 .0270 .0429 .0640 .0911 .1250

p



(1 )np pσ = −(1 )np pσ = −

E(x) = μ = np

Var(x) = σ 2 = np(1 − p)

� Expected Value

� Variance

� Standard Deviation


3(.1)(.9) .52 employeesσ = =3(.1)(.9) .52 employeesσ = =

E(x) = μ = 3(.1) = .3 employees out of 3

Var(x) = σ 2 = 3(.1)(.9) = .27

� Expected Value

� Variance

� Standard Deviation

Chapter 5 Discrete Probability Distributions - Texas A&M...

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