Chapter 5: DISCRETE RANDOM VARIABLES AND THEIR PROBABILITY DISTRIBUTIONS
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RANDOM VARIABLES Discrete Random Variable ر� ي����� غ�� طع���������مت��� ق� �ى مت��� و���ائ� عش��� Continuous Random Variable ر� ي����� غ�� �ى مس��ت�م���ر�مت��� و���ائ� عش���
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RANDOM VARIABLES cont.Table 5.1 Frequency and Relative Frequency
Distribution of the Number of Vehicles Owned by Families. Number of Vehicles Owned
Frequency Relative Frequency
01234
30470850490160
30/2000 = .015470/2000 = .235850/2000 = .425490/2000 = .245160/2000 = .080
N = 2000 Sum = 1.000
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RANDOM VARIABLES cont. Definition A random variable is a variable whose value is determined by the outcome of a random experiment.
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Discrete Random Variable
Definition A random variable that assumes countable values is called a discrete random variable. م�� مع������دودة� ي� � ق��������
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Examples of discrete random variables
1. The number of cars sold at a dealership during a given month
2. The number of houses in a certain block3. The number of fish caught on a fishing
trip4. The number of complaints received at the
office of an airline on a given day5. The number of customers who visit a bank
during any given hour6. The number of heads obtained in three
tosses of a coin
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Continuous Random Variable
Definition A random variable that can assume any value contained in one or more intervals is called a continuous random variable.
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Continuous Random Variable cont.
2000
Every point on this line represents a possible value of x that denotesthe life of a battery. There are an infinite num ber of points on this line.The values represented by points on this line are uncountable.
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Examples of continuous random variables
1. The height of a person2. The time taken to complete an examination3. The amount of milk in a gallon (note that
we do not expect a gallon to contain exactly one gallon of milk but either slightly more or slightly less than a gallon.)
4. The weight of a fish5. The price of a house
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PROBABLITY DISTRIBUTION OF A DISCRETE RANDOM VARIABLE
Definition The probability distribution of a discrete
random variable lists all the possible values that the random variable can assume and their corresponding probabilities.
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Example 5-1 Recall the frequency and relative frequency distributions of the number of vehicles owned by families given in Table 5.1. That table is reproduced below as Table 5.2. Let x be the number of vehicles owned by a randomly selected family. Write the probability distribution of x.
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Table 5.2 Frequency and Relative Frequency Distributions of the Vehicles Owned by Families Number of Vehicles Owned Frequency
Relative Frequency
01234
30470850490160
30/2000 = .015470/2000 = .235850/2000 = .425490/2000 = .245160/2000 = .080
N = 2000 Sum = 1.000
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Solution 5-1Table 5.3 Probability Distribution of the
Number of Vehicles Owned by Families
Number of Vehicles Owned
x
ProbabilityP(x)
01234
.015
.235
.425
.245
.080ΣP(x) = 1.000
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Two Characteristics of a Probability Distribution
The probability distribution of a discrete random variable possesses the following two characteristics.1. 0 ≤ P (x) ≤ 1 for each value of x2. ΣP (x) = 1
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Figure 5.1 Graphical presentation of the probability distribution of Table 5.3.
00.050.10.150.20.250.30.350.40.45
x
P(x)
0 1 2 3 4
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Each of the following tables lists certain values of x and their probabilities. Determine whether or not each table represents a valid probability distribution.
Example 5-2
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Example 5-2
a) x P(x) b) x P(x)0123
.08
.11
.39
.27
2345
.25
.34
.28
.13
c) x P(x)789
.70
.50-.20
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Example 5-3 The following table lists the probability distribution of the number of breakdowns per week for a machine based on past data.Breakdowns per week 0 1 2 3
Probability .15 .20 .35 .30
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Example 5-3a) Present this probability
distribution graphically.b) Find the probability that the
number of breakdowns for this machine during a given week is
i. exactly 2 ii. 0 to 2iii.more than 1 iv. at most 1
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Solution 5-3 Let x denote the number of breakdowns for this machine during a given week. Table 5.4 lists the probability distribution of x.
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Table 5.4 Probability Distribution of the Number of Breakdowns
x P(x)0123
.15
.20
.35
.30ΣP(x) = 1.00
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Figure 5.2 Graphical presentation of the probability distribution of Table 5.4.
00.050.10.150.20.250.30.350.4
x
P(x)
0 1 2 3
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Solution 5-3 (b) i. P (exactly 2 breakdowns) = P (x = 2) = .35ii. P (0 to 2 breakdowns) = P (0 ≤ x ≤ 2)
= P (x = 0) + P (x = 1) + P (x = 2) = .15 + .20 + .35 = .70
iii. P (more then 1 breakdown) = P (x > 1) = P (x = 2) + P (x = 3) = .35 +.30 = .65
iv. P (at most one breakdown) = P (x ≤ 1) = P (x = 0) + P (x = 1) = .15 + .20 = .35
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Example 5-4 According to a survey, 60% of all students at a large university suffer from math anxiety. Two students are randomly selected from this university. Let x denote the number of students in this sample who suffer from math anxiety. Develop the probability distribution of x.
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Figure 5.3 Tree diagram.
N.40
M.60
N.40
M.60
N
.40
.60M
P(NN) = (.40)(.40) = .16
P(NM) = (.40)(.60) = .24
P(MN) = (.60)(.40) = .24
P(MM) = (.60)(.60) = .36
S econd S tudent Final OutcomesFirst S tudent
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Solution 5-4 Let us define the following two events: N = the student selected does not suffer from math anxiety M = the student selected suffers from math anxiety
P (x = 0) = P(NN) = .16P (x = 1) = P(NM or MN) = P(NM) + P(MN)
= .24 + .24 = .48
P (x = 2) = P(MM) = .36
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Table 5.5 Probability Distribution of the Number of Students with Math Anxiety in a Sample of Two Students
x P(x)012
.16
.48
.36ΣP(x) = 1.00
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MEAN OF A DISCRETE RANDOM VARIABLE
The mean of a discrete variable x is the value that is expected to occur per repetition, on average, if an experiment is repeated a large number of times. It is denoted by µ and calculated as
µ = Σx P (x)
The mean of a discrete random variable x is also called its expected value and is denoted by E (x); that is,
E (x) = Σx P (x)
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Example 5-5 Recall Example 5-3. The probability distribution Table 5.4 from that example is reproduced on the next slide. In this table, x represents the number of breakdowns for a machine during a given week, and P (x) is the probability of the corresponding value of x.
Find the mean number of breakdown per week for this machine.
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Table 5.4 Probability Distribution of the Number of Breakdowns
x P(x)0123
.15
.20
.35
.30ΣP(x) = 1.00
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Table 5.6 Calculating the Mean for the Probability Distribution of Breakdowns
Solution 5-5x P(x) xP(x)0123
.15
.20
.35
.30
0(.15) = .001(.20) = .202(.35) = .703(.30) = .90ΣxP(x) = 1.80
The mean is µ = Σx P (x) = 1.80
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STANDARD DEVIATION OF A DISCRETE RANDOM VARIABLE
The standard deviation of a discrete random variable x measures the spread of its probability distribution and is computed as
22 )( xPx
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Example 5-6 Baier’s Electronics manufactures computer parts that are supplied to many computer companies. Despite the fact that two quality control inspectors at Baier’s Electronics check every part for defects before it is shipped to another company, a few defective parts do pass through these inspections undetected. Let x denote the number of defective computer parts in a shipment of 400. The following table gives the probability distribution of x.
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Solution 5-6Table 5.7 Computations to Find the Standard
Deviationx P(x) xP(x) x² x²P(x)
012345
.02
.20
.30
.30
.10
.08
.00
.20
.60
.90
.40
.40
0 1 4 91625
.00 .201.202.701.602.00
ΣxP(x) = 2.50 Σx²P(x) = 7.70
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Solution 5-6
partscomputer defective 204.1 45.1)50.2(70.7
400in partscomputer defective 50.2222
xPxσ
xPx
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Example 5-7 Loraine Corporation is planning to market a new makeup product. According to the analysis made by the financial department of the company, it will earn an annual profit of $4.5 million if this product has high sales and an annual profit of $ 1.2 million if the sales are mediocre, and it will lose $2.3 million a year if the sales are low. The probabilities of these three scenarios are .32, .51 and .17 respectively.
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Example 5-7a) Let x be the profits (in
millions of dollars) earned per annum by the company from this product. Write the probability distribution of x.
b) Calculate the mean and the standard deviation of x.
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Solution 5-7a) The following table lists the
probability distribution of x. x P(x) 4.5 1.2-2.3
0.320.510.17
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Table 5.8 Computations to Find the Mean and Standard Deviation
x P(x) xP(x) x² x²P(x) 4.5 1.2-2.3
.32
.51
.17
1.440.612-.391
20.25
1.445.29
6.48000.73440.8993
Σ xP(x) = 1.661
Σ x²P(x) = 8.1137
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Factorials Definition The symbol n!, read as “n factorial”, represents the product of all the integers from n to 1. in other words,
n! = n(n - 1)(n – 2)(n – 3). . . 3 . 2 . 1 By definition,
0! = 1
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Solutiona) 7! = 7 · 6 · 5 · 4 · 3 · 2 · 1 = 5040b) 10! = 10 · 9 · 8 · 7 · 6 · 5 · 4 · 3
· 2 · 1 = 3,628,800c) (12 – 4)! = 8! = 8 · 7 · 6 · 5 · 4 ·
3 · 2 · 1 = 40,320d) (5 – 5)! = 0! = 1
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Combinations Definition Combinations give the number of ways x elements can be selected from n elements. The notation used to denote the total number of combinations is
which is read as “the number of combinations of n elements selected x at a time.”
xnC
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Combinations cont.
xnCn denotes the total number of elements
= the number of combinations of n elements selected x at a timex denotes the number of
elements selected per selection
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Combinations cont. Number of Combinations The number of combinations for selecting x from n distinct elements is given by the formula
)!(!!
xnxnC xn
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Example 5-13 An ice cream parlor has six flavors of ice cream. Kristen wants to buy two flavors of ice cream. If she randomly selects two flavors out of six, how many combinations are there?
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Solution 5-13
15123412123456
!4!2!6
)!26(!2!6
26
C
Thus, there are 15 ways for Kristin to select two ice cream flavors out of six.
n = 6 x = 2
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Example 5-14 Three members of a jury will be randomly selected from five people. How many different combinations are possible?
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Using the Table of Combinations
Example 5-15 Marv & Sons advertised to hire a financial analyst. The company has received applications from 10 candidates who seem to be equally qualified. The company manager has decided to call only 3 of these candidates for an interview. If she randomly selects 3 candidates from the 10, how many total selections are possible?
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THE BINOMIAL PROBABILITY DISTRIBUTION
The Binomial Experiment The Binomial Probability Distribution and binomial Formula
Using the Table of Binomial Probabilities
Probability of Success and the Shape of the Binomial Distribution
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The Binomial Experiment
Conditions of a Binomial ExperimentA binomial experiment must satisfy thefollowing four conditions.
1. There are n identical trials.2. Each trail has only two possible
outcomes.3. The probabilities of the two outcomes
remain constant.4. The trials are independent.
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The Binomial Probability Distribution and Binomial Formula
For a binomial experiment, the probability of exactly x successes in n trials is given by the binomial formula
where n = total number of trials p = probability of success q = 1 – p = probability of failure x = number of successes in n trials n - x = number of failures in n trials
xnxxn qpCxP )(
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Example 5-18 Five percent of all VCRs manufactured by a large electronics company are defective. A quality control inspector randomly selects three VCRs from the production line.What is the probability that exactly one of these three VCRs are defective?
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Figure 5.4 Tree diagram for selecting three VCRs.
DDD
DDG
DGD
DGGGDD
GDGGGD
GGG
First VCR Second VCR Third VCR
D.05
G
.95
D
D
D
D
D
.05
.05
.05
.05
.05
.05
D
G
G
G
G
G
G
.95
.95
.95
.95
.95
.95
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Solution 5-18 Let
D = a selected VCR is defectiveG = a selected VCR is good
P (DGG ) = P (D )P (G )P (G ) = (.05)(.95)(.95) = .0451 P (GDG ) = P (G )P (D )P (G )
= (.95)(.05)(.95) = .0451 P (GGD ) = P (G )P (G )P (D )
= (.95)(.95)(.05) = .0451
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Solution 5-18Therefore, P (1 VCR is defective in 3) = P (DGG or GDG or GGD ) = P (DGG ) + P (GDG ) + P (GGD ) = .0451 + .0451 + .0451 = .1353
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Solution 5-18 n = total number of trials = 3 VCRs
x = number of successes = number of defective VCRs = 1
n – x = 3 - 1 = 2 p = P (success) = .05
q = P (failure) = 1 – p = .95
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Solution 5-18 Therefore, the probability of selecting exactly one defective VCR.
The probability .1354 is slightly different from the earlier calculation .1353 because of rounding.
1354.)9025)(.05)(.3()95(.)05(.)1( 2113 CxP
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Example 5-19 At the Express House Delivery Service, providing high-quality service to customers is the top priority of the management. The company guarantees a refund of all charges if a package it is delivering does not arrive at its destination by the specified time. It is known from past data that despite all efforts, 2% of the packages mailed through this company do not arrive at their destinations within the specified time. Suppose a corporation mails 10 packages through Express House Delivery Service on a certain day.
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Example 5-19a) Find the probability that exactly
1 of these 10 packages will not arrive at its destination within the specified time.
b) Find the probability that at most 1 of these 10 packages will not arrive at its destination within the specified time.
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Solution 5-19 n = total number of packages mailed = 10
p = P (success) = .02 q = P (failure) = 1 – .02 = .98
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Solution 5-19 x = number of successes = 1 n – x = number of failures =
10 – 1 = 9
1667.)83374776)(.02)(.10(
)98(.)02(.)!110(!1!10)98(.)02(.)1( 9191
110
CxP
a)
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Solution 5-19 b)
.9838 .1667.8171 .83374776)(10)(.02)(707281)(1)(1)(.81
)98(.)02(.)98(.)02(. )1()0()1(
91110
100010
CCxPxPxP
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Example 5-20 According to an Allstate Survey, 56% of Baby Boomers have car loans and are making payments on these loans (USA TODAY, October 28, 2002). Assume that this result holds true for the current population of all Baby Boomers. Let x denote the number in a random sample of three Baby Boomers who are making payments on their car loans. Write the probability distribution of x and draw a bar graph for this probability distribution.
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Solution 5-20 n = total Baby boomers in the sample = 3
p = P (a Baby Boomer is making car loan payments) = .56
q = P (a Baby Boomer is not making car loan payments) = 1 - .56 = .44
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Solution 5-20
0852.)085184)(.1)(1()44.0()56(.)0( 3003 CxP
3252.)1936)(.56)(.3()44(.)56(.)1( 2113 CxP
4140.)44)(.3136)(.3()44(.)56(.)2( 1223 CxP
1756.)1)(175616)(.1()44(.)56(.)3( 0333 CxP
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Figure 5.5 Bar graph of the probability distribution of x.
00.050.10.150.20.250.30.350.40.45
x
P(x)
0 1 2 3
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Using the Table of Binomial Probabilities
Example 5-21 According to a 2001 study of college students by Harvard University’s School of Public health, 19.3% of those included in the study abstained from drinking (USA TODAY, April 3, 2002). Suppose that of all current college students in the United States, 20% abstain from drinking. A random sample of six college students is selected.
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Example 5-21Using Table IV of Appendix C, answer the
following.a) Find the probability that exactly three college
students in this sample abstain from drinking.b) Find the probability that at most two college
students in this sample abstain from drinking.c) Find the probability that at least three
college students in this sample abstain from drinking.
d) Find the probability that one to three college students in this sample abstain from drinking.
e) Let x be the number of college students in this sample who abstain from drinking. Write the probability distribution of x and draw a bar graph for this probability distribution.
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Table 5.11 Determining P (x = 3) for n = 6 and p = .20
pn x .05 .10 .20 … .956 0
123456
.7351
.2321
.0305
.0021
.0001
.0000
.0000
.5314
.3543
.0984
.0146
.0012
.0001
.0000
.2621
.3932
.2458
.0819
.0154
.0015
.0001
…………………
.0000
.0000
.0001
.0021
.0305
.2321
.7351
n = 6
p =.20
P (x = 3) = .0819
x = 3
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Solution 5-21a) P (x = 3) = .0819b) P (at most 2) = P (0 or 1 or 2)
= P (x = 0) + P (x = 1) + P (x = 2) = .2621 + .3932 + .2458 = .9011
c) P (at least 3) = P(3 or 4 or 5 or 6) = P (x = 3) + P (x = 4) + P (x =5) + P (x = 6) = .0819 + .0154 + .0015 + .0001 = .0989
d) P (1 to 3) = P (x = 1) + P (x = 2) + P (x = 3) = .3932 + .2458 + .0819 = .7209
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Table 5.13 Probability Distribution of x for n = 6 and p= .20
x P(x)0123456
.2621
.3932
.2458
.0819
.0154
.0015
.0001
79
Figure 5.6 Bar graph for the probability distribution of x.
00.050.10.150.20.250.30.350.40.45
x
P(x)
0 1 2 3 4 5 6
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Probability of Success and the Shape of the Binomial Distribution
1. The binomial probability distribution is symmetric if p = .50 x P(x)
01234
.0625
.2500
.3750
.2500
.0625
Table 5.14 Probability
Distribution of x for n = 4 and p = .50
81
Figure 5.7 Bar graph from the probability distribution of Table 5.14.
0
0.1
0.2
0.3
0.4
x
P(x)
0 1 2 3 4
82
Probability of Success and the Shape of the Binomial Distribution cont.
2. The binomial probability distribution is skewed to the right if p is less than .50.
x P(x)01234
.2401
.4116
.2646
.0756
.0081
Table 5.15 Probability
Distribution of x for n = 4 and p = .30
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Figure 5.8 Bar graph for the probability distribution of Table 5.15.
0
0.1
0.2
0.3
0.4
0.5
x
P(x)
0 1 2 3 4
84
Probability of Success and the Shape of the Binomial Distribution cont.
3. The binomial probability distribution is skewed to the left if p is greater than .50.
x P(x)01234
.0016
.0256
.1536
.4096
.4096
Table 5.16
Probability
Distribution of x for n =4 and p = .80
85
Figure 5.9 Bar graph for the probability distribution of Table 5.16.
0
0.1
0.2
0.3
0.4
0.5
x
P(x)
0 1 2 3 4
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Mean and Standard Deviation of the Binomial Distribution
The mean and standard deviation of a binomial distribution are
where n is the total number of trails, p is the probability of success, and q is the probability of failure.
npqnp and
87
Example 5-22 In a Martiz poll of adult drivers conducted in July 2002, 45% said that they “often” or “sometimes” eat or drink while driving (USA TODAY, October 23, 2002). Assume that this result is true for the current population of all adult drivers. A sample of 40 adult drivers is selected. Let x be the number of drivers in this sample who “often” or “sometimes” eat or drink while driving. Find the mean and standard deviation of the probability distribution of x.
89
THE HYPERGEOMETRIC PROBABILITY DISTRIBUTION
Let N = total number of elements in the population
r = number of successes in the population
N – r = number of failures in the population n = number of trials (sample size) x = number of successes in n trials n – x = number of failures in n trials
90
THE HYPERGEOMETRIC PROBABILITY DISTRIBUTION
The probability of x successes in n trials is given by
nN
xnrNxr
CCCxP )(
91
Example 5-23 Brown Manufacturing makes auto parts that are sold to auto dealers. Last week the company shipped 25 auto parts to a dealer. Later on, it found out that five of those parts were defective. By the time the company manager contacted the dealer, four auto parts from that shipment have already been sold. What is the probability that three of those four parts were good parts and one was defective?
92
Solution 5-23
4506.650,12)5)(1140(
)!425(!4!25
)!15(!1!5
)!320(!3!20
)3(425
15320
CCC
CCCxPnN
xnrNxr
Thus, the probability that three of the four parts sold are good and one is defective is .4506.
93
Example 5-24 Dawn Corporation has 12 employees who
hold managerial positions. Of them, seven are female and five are male. The company is planning to send 3 of these 12 managers to a conference. If 3 managers are randomly selected out of 12,a) Find the probability that all 3 of them are
femaleb) Find the probability that at most 1 of them is
a female
94
Solution 5-24
1591.220)1)(35( )3(
312
0537
CCC
CCCxPnN
xnrNxr
Thus, the probability that all three of managers selected re female is .1591.
(a)
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Solution 5-24
3637.3182.0455.)1()0()1(
3182.220)10)(7( )1(
0455.220)10)(1( )0(
312
2517
312
3507
xPxPxPC
CCC
CCxP
CCC
CCCxP
nN
xnrNxr
nN
xnrNxr(b)
96
THE POISSON PROBABILITY DISTRIBUTION
Using the Table of Poisson probabilities
Mean and Standard Deviation of the Poisson Probability Distribution
97
THE POISSON PROBABILITY DISTRIBUTION cont.
Conditions to Apply the Poisson Probability Distribution
The following three conditions must be satisfied to apply the Poisson probability distribution.1. x is a discrete random variable.2. The occurrences are random.3. The occurrences are independent.
98
Examples1. The number of accidents that occur on
a given highway during a one-week period.
2. The number of customers entering a grocery store during a one –hour interval.
3. The number of television sets sold at a department store during a given week.
99
THE POISSON PROBABILITY DISTRIBUTION cont.
Poisson Probability Distribution Formula
According to the Poisson probability distribution, the probability of x occurrences in an interval is
where λ is the mean number of occurrences in that interval and the value of e is approximately 2.71828.
!)(xexP
x
100
Example 5-25 On average, a household receives 9.5 telemarketing phone calls per week. Using the Poisson distribution formula, find the probability that a randomly selected household receives exactly six telemarketing phone calls during a given week.
102
Example 5-26 A washing machine in a laundromat breaks down an average of three times per month. Using the Poisson probability distribution formula, find the probability that during the next month this machine will have
a) exactly two breakdownsb) at most one breakdown
103
Solution 5-26
.1992 .1494 .0498 1
)04978707)(.3(1
)04978707)(.1( !1)3(
!0)3()1()0(
)(
2240.2)04978707)(.9(
!2)3()2(
)(
3130
32
eexPxP
b
exP
a
104
Example 5-27 Cynthia’s Mail Order Company provides free examination of its products for seven days. If not completely satisfied, a customer can return the product within that period and get a full refund. According to past records of the company, an average of 2 of every 10 products sold by this company are returned for a refund. Using the Poisson probability distribution formula, find the probability that exactly 6 of the 40 products sold by this company on a given day will be returned for a refund.
106
Using the Table of Poisson Probabilities
Example 5-28 On average, two new accounts are opened per day at an Imperial Saving Bank branch. Using the Poisson table, find the probability that on a given day the number of new accounts opened at this bank will bea) exactly 6 b) at most 3 c) at least 7
107
Table 5.17 Portion of Table of Poisson Probabilities for λ = 2.0
λx 1.1 1.2 … 2.00123456789
.1353
.2707
.2707
.1804
.0902
.0361
.0120
.0034
.0009
.0002
x = 6 P (x = 6)
λ = 2.0
108
Solution 5-28a) P (x = 6) = .0120b) P (at most 3) =P (x = 0) + P (x = 1) + P (x =
2) + P (x = 3) =.1353 +.2707 + .2707 + .1804 = .8571
c) P (at least 7) = P (x = 7) + P (x = 8) + P (x = 9) = .0034 + .0009 + .0002 = .0045
110
Example 5-29 An auto salesperson sells an average
of .9 car per day. Let x be the number of cars sold by this salesperson on any given day. Using the Poisson probability distribution table,a) Write the probability distribution of x. b) Draw a graph of the probability
distribution.c) Find the mean, variance, and standard
deviation.
111
Table 5.18 Probability Distribution of x for λ = .9
x P (x)0123456
.4066
.3659
.1647
.0494
.0111
.0020
.0003
Solution 5-29 a
112
Figure 5.10 Bar graph for the probability distribution of Table 5.18.
00.050.10.150.20.250.30.350.40.45
x
P(x)
0 1 2 3 4 5 6
Solution 5-29 b