chapter 5 discrete probability distributions 1 copyright © 2012 the mcgraw-hill companies, inc....
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Chapter 5
Discrete Probability Distributions
1Copyright © 2012 The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Outline
5Discrete Probability Distributions
1.1
Descriptive and Inferential Statistics
Slide 2Copyright © 2012 The McGraw-Hill Companies, Inc.
5-1 Probability Distributions5-2 Mean, Variance, Standard Deviation, and
Expectation5-3 The Binomial Distribution5-4 Other Types of Distributions (Optional)
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Objectives
5Discrete Probability Distributions
1.1
Descriptive and inferential statistics1 Construct a probability distribution for a random
variable.2 Find the mean, variance, standard deviation, and
expected value for a discrete random variable.3 Find the exact probability for X successes in n trials
of a binomial experiment.4 Find the mean, variance, and standard deviation for
the variable of a binomial distribution.
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Objectives
5Discrete Probability Distributions
1.1
Descriptive and inferential statistics5 Find probabilities for outcomes of variables, using
the Poisson, hypergeometric, and multinomial distributions.
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5.1 Probability Distributions• A random variable is a variable whose values are
determined by chance.
• A discrete probability distribution consists of the values a random variable can assume and the corresponding probabilities of the values.
• The sum of the probabilities of all events in a sample space add up to 1. Each probability is between 0 and 1, inclusively.
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Example 5-1: Rolling a Die
Construct a probability distribution for rolling a single die.
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Example 5-2: Tossing CoinsRepresent graphically the probability distribution for the sample space for tossing three coins..
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5-2 Mean, Variance, Standard Deviation, and Expectation
MEAN: X P X
2 2 2
VARIANCE:
X P X
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Rounding Rule The mean, variance, and standard deviation should be rounded to one more decimal place than the outcome X.
When fractions are used, they should be reduced to lowest terms.
Mean, Variance, Standard Deviation, and Expectation
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Example 5-5: Rolling a DieFind the mean of the number of spots that appear when a die is tossed.
.
X P X 1 1 1 1 1 16 6 6 6 6 61 2 3 4 5 6
216 3.5
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Example 5-8: Trips of 5 Nights or MoreThe probability distribution shown represents the number of trips of five nights or more that American adults take per year. (That is, 6% do not take any trips lasting five nights or more, 70% take one trip lasting five nights or more per year, etc.) Find the mean.
.
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Example 5-8: Trips of 5 Nights or More
X P X
0 0.06 1 0.70 2 0.20
3 0.03 4 0.01
1.2
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Example 5-9: Rolling a DieCompute the variance and standard deviation for the probability distribution in Example 5–5.
.
2 2 2X P X
2 2 2 2 21 1 1 16 6 6 6
22 21 16 6
1 2 3 4
5 6 3.5
2 2.9 , 1.7
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Example 5-11: On Hold for Talk RadioA talk radio station has four telephone lines. If the host is unable to talk (i.e., during a commercial) or is talking to a person, the other callers are placed on hold. When all lines are in use, others who are trying to call in get a busy signal. The probability that 0, 1, 2, 3, or 4 people will get through is shown in the distribution. Find the variance and standard deviation for the distribution.
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Example 5-11: On Hold for Talk Radio
2 2 2 2
22 2
0 0.18 1 0.34 2 0.23
3 0.21 4 0.04 1.6
2 1.2 , 1.1
0 0.18 1 0.34 2 0.23
3 0.21 4 0.04 1.6
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Example 5-11: On Hold for Talk RadioA talk radio station has four telephone lines. If the host is unable to talk (i.e., during a commercial) or is talking to a person, the other callers are placed on hold. When all lines are in use, others who are trying to call in get a busy signal.
Should the station have considered getting more phone lines installed?
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Example 5-11: On Hold for Talk RadioNo, the four phone lines should be sufficient. The mean number of people calling at any one time is 1.6. Since the standard deviation is 1.1, most callers would be accommodated by having four phone lines because µ + 2 would be
1.6 + 2(1.1) = 1.6 + 2.2 = 3.8. Very few callers would get a busy signal since at least 75% of the callers would either get through or be put on hold. (See Chebyshev’s theorem in Section 3–2.)
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Expectation• The expected value, or expectation, of a
discrete random variable of a probability distribution is the theoretical average of the variable.
• The expected value is, by definition, the mean of the probability distribution.
E X X P X
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Example 5-13: Special DieA special six-sided die is made in which 3 sides have 6 spots, 2 sides have 4 spots, and 1 side has 1 spot.
If the die is rolled, find the expected value of the number of spots that will occur.
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