Sfs4e ppt 08

Copyright © 2014, 2013, 2010 and 2007 Pearson Education, Inc.
Chapter
Sampling
Distributions
8

Section
Distribution of the
Sample Mean
8.1

Copyright © 2014, 2013, 2010 and 2007 Pearson Education, Inc.8-3
Objectives
1. Describe the distribution of the sample mean:
normal population
2. Describe the distribution of the sample mean:
nonnormal population

Statistics such as are random variables since
their value varies from sample to sample. As
such, they have probability distributions
associated with them. In this chapter we focus
on the shape, center and spread of statistics such
as .
x
x

The sampling distribution of a statistic is a
probability distribution for all possible values of
the statistic computed from a sample of size n.
The sampling distribution of the sample mean
is the probability distribution of all possible
values of the random variable computed from
a sample of size n from a population with mean
μ and standard deviation σ.
x
x

Illustrating Sampling Distributions
Step 1: Obtain a simple random sample of size n.

Step 2: Compute the sample mean.

Step 2: Compute the sample mean.
Step 3: Assuming we are sampling from a finite
population, repeat Steps 1 and 2 until all
simple random samples of size n have
been obtained.

Objective 1
• Describe the Distribution of the Sample Mean:
Normal Population

The weights of pennies minted after 1982 are
approximately normally distributed with mean 2.46
grams and standard deviation 0.02 grams.
Approximate the sampling distribution of the
sample mean by obtaining 200 simple random
samples of size n = 5 from this population.
Parallel Example 1: Sampling Distribution of the Sample
Mean-Normal Population

The data on the following slide represent the
sample means for the 200 simple random samples
of size n = 5.
For example, the first sample of n = 5 had the
following data:
2.493 2.466 2.473 2.492 2.471
Note: = 2.479 for this samplex

Sample Means for Samples of Size n = 5

The mean of the 200 sample means is 2.46, the
same as the mean of the population.
The standard deviation of the sample means is
0.0086, which is smaller than the standard
deviation of the population.
The next slide shows the histogram of the
sample means.

What role does n, the sample size, play in the
standard deviation of the distribution of the
sample mean?

What role does n, the sample size, play in the
standard deviation of the distribution of the
sample mean?
As the size of the sample increases, the standard
deviation of the distribution of the sample mean
decreases.

• Approximate the sampling distribution of the
sample mean by obtaining 200 simple random
samples of size n = 20 from the population of
weights of pennies minted after 1982 (μ = 2.46
grams and σ = 0.02 grams)
Parallel Example 2: The Impact of Sample Size on Sampling
Variability

The mean of the 200 sample means for n = 20 is still 2.46,
but the standard deviation is now 0.0045 (0.0086 for n =
5). As expected, there is less variability in the distribution
of the sample mean with n =20 than with n = 5.

Suppose that a simple random sample of size n is
drawn from a large population with mean μ and
standard deviation σ. The sampling distribution of
will have mean and standard deviation
The standard deviation of the sampling distribution
of is called the standard error of the mean and
is denoted .
The Mean and Standard Deviation of the
Sampling Distribution of
x
x
σx
σx =
σ
n
.
µx = µ
x

The Shape of the Sampling
Distribution of If X is Normalx
If a random variable X is normally distributed,
the distribution of the sample mean is
normally distributed.
x

The weights of pennies minted after 1982 are
approximately normally distributed with mean
2.46 grams and standard deviation 0.02 grams.
What is the probability that in a simple random
sample of 10 pennies minted after 1982, we
obtain a sample mean of at least 2.465 grams?
Parallel Example 3: Describing the Distribution of the
Sample Mean

• is normally distributed with =2.46 and .
• .
• P(Z > 0.79) = 1 – 0.7852
= 0.2148.
Solution
x µx
σx =
0.02
10
=0.0063
Z=
2.465−2.46
0.0063
=0.79

Objective 2
• Describe the Distribution of the Sample Mean:
Nonnormal Population

The following table and histogram
give the probability distribution
for rolling a fair die:
μ = 3.5, σ = 1.708
Note that the population distribution is NOT normal
Face on Die Relative Frequency
1 0.1667
2 0.1667
3 0.1667
4 0.1667
5 0.1667
6 0.1667
Parallel Example 4: Sampling from a Population that is Not
Normal

Estimate the sampling distribution of by obtaining
200 simple random samples of size n = 4 and
calculating the sample mean for each of the 200
samples. Repeat for n = 10 and 30.
Histograms of the sampling distribution of the sample
mean for each sample size are given on the next slide.
x

Key Points from Example 4
• The mean of the sampling distribution is equal to
the mean of the parent population and the
standard deviation of the sampling distribution of
the sample mean is regardless of the sample
size.
• The Central Limit Theorem: the shape of the
distribution of the sample mean becomes
approximately normal as the sample size n
increases, regardless of the shape of the
population.
σ
n

Parallel Example 5: Using the Central Limit Theorem
Suppose that the mean time for an oil change at a “10-minute
oil change joint” is 11.4 minutes with a standard deviation
of 3.2 minutes.
(a) If a random sample of n = 35 oil changes is selected,
describe the sampling distribution of the sample mean.
(b) If a random sample of n = 35 oil changes is selected, what
is the probability the mean oil change time is less than 11
minutes?

of 3.2 minutes.
Solution: is approximately normally distributed
with mean = 11.4 and std. dev. = .3.2
35
= 0.5409
x

of 3.2 minutes.
(b) If a random sample of n = 35 oil changes is selected, what
is the probability the mean oil change time is less than 11
minutes?
Solution: is approximately normally distributed
with mean = 11.4 and std. dev. = .
Solution: , P(Z < –0.74) = 0.23.
3.2
35
= 0.5409
x
Z =
11−11.4
0.5409
= −0.74

Section
Distribution of the
Sample
Proportion
8.2

Objectives
1. Describe the sampling distribution of a
sample proportion
2. Compute probabilities of a sample proportion

Objective 1
• Describe the Sampling Distribution of a
Sample Proportion

Point Estimate of a Population
Proportion
Suppose that a random sample of size n is obtained
from a population in which each individual either
does or does not have a certain characteristic. The
sample proportion, denoted (read “p-hat”) is
given by
where x is the number of individuals in the sample
with the specified characteristic. The sample
proportion is a statistic that estimates the
population proportion, p.
ˆp
ˆp =
x
n
ˆp

In a Quinnipiac University Poll conducted in May of
2008, 1745 registered voters nationwide were asked
whether they approved of the way George W. Bush
is handling the economy. 349 responded “yes”.
Obtain a point estimate for the proportion of
registered voters who approve of the way George
W. Bush is handling the economy.
Parallel Example 1: Computing a Sample Proportion

In a Quinnipiac University Poll conducted in May of
2008, 1,745 registered voters nationwide were asked
whether they approved of the way George W. Bush
is handling the economy. 349 responded “yes”.
Obtain a point estimate for the proportion of
registered voters who approve of the way George
W. Bush is handling the economy.
Parallel Example 1: Computing a Sample Proportion
Solution: ˆp =
349
1745
= 0.2

According to a Time poll conducted in June of
2008, 42% of registered voters believed that gay
and lesbian couples should be allowed to marry.
Describe the sampling distribution of the sample
proportion for samples of size n = 10, 50, 100.
Note: We are using simulations to create the histograms on the
following slides.
Parallel Example 2: Using Simulation to Describe the
Distribution of the Sample Proportion

Key Points from Example 2
• Shape: As the size of the sample, n, increases,
the shape of the sampling distribution of the
sample proportion becomes approximately
normal.
• Center: The mean of the sampling distribution
of the sample proportion equals the population
proportion, p.
• Spread: The standard deviation of the
sampling distribution of the sample proportion
decreases as the sample size, n, increases.

For a simple random sample of size n with
population proportion p:
• The shape of the sampling distribution of is
approximately normal provided np(1 – p) ≥ 10.
• The mean of the sampling distribution of is
• The standard deviation of the sampling distribution
of is
ˆp
ˆp
µˆp =p
ˆp
σˆp =
p(1−p)
n
ˆp

• The model on the previous slide requires that the
sampled values are independent. When sampling
from finite populations, this assumption is verified
by checking that the sample size n is no more than
5% of the population size N (n ≤ 0.05N).
• Regardless of whether np(1 – p) ≥ 10 or not, the
mean of the sampling distribution of is p, and
the standard deviation is
ˆp
σˆp =
p(1−p)
n
ˆp

According to a Time poll conducted in June of 2008,
42% of registered voters believed that gay and
lesbian couples should be allowed to marry.
Suppose that we obtain a simple random sample of
50 voters and determine which voters believe that
gay and lesbian couples should be allowed to marry.
Describe the sampling distribution of the sample
proportion for registered voters who believe that gay
and lesbian couples should be allowed to marry.
Parallel Example 3: Describing the Sampling Distribution of
the Sample Proportion

Solution
The sample of n = 50 is smaller than 5% of the
population size (all registered voters in the U.S.).
Also, np(1 – p) = 50(0.42)(0.58) = 12.18 ≥ 10.
The sampling distribution of the sample proportion
is therefore approximately normal with mean=0.42
and standard deviation =
(Note: this is very close to the standard deviation of 0.072 found
using simulation in Example 2.)
0.42(1− 0.42)
50
= 0.0698

Objective 2
• Compute Probabilities of a Sample Proportion

According to the Centers for Disease Control and
Prevention, 18.8% of school-aged children, aged 6-11 years,
were overweight in 2004.
(a)In a random sample of 90 school-aged children, aged 6-
11 years, what is the probability that at least 19% are
overweight?
(b)Suppose a random sample of 90 school-aged children,
aged 6-11 years, results in 24 overweight children. What
might you conclude?
Parallel Example 4: Compute Probabilities of a Sample
Proportion

• n = 90 is less than 5% of the population size
• np(1 – p) = 90(.188)(1 – .188) ≈ 13.7 ≥ 10
• is approximately normal with mean=0.188 and
standard deviation =
(a) In a random sample of 90 school-aged children, aged
6-11 years, what is the probability that at least 19%
are overweight?
Solution
ˆp
(0.188)(1−0.188)
90
=0.0412
, P(Z > 0.05)=1 – 0.5199=0.4801Z =
0.19 − 0.188
0.0412
= 0.0485

• is approximately normal with mean = 0.188 and
standard deviation = 0.0412
(b) Suppose a random sample of 90 school-aged
children, aged 6-11 years, results in 24 overweight
children. What might you conclude?
Solution
ˆp
,
P(Z > 1.91) = 1 – 0.9719 = 0.028.
We would only expect to see about 3 samples in 100
resulting in a sample proportion of 0.2667 or more.
This is an unusual sample if the true population
proportion is 0.188.
ˆp =
24
90
= 0.2667 Z =
0.2667 − 0.188
0.0412
= 1.91

Sfs4e ppt 08

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Sfs4e ppt 08