Transcript Sampling Distributions

Sampling Distributions
•Numerical
descriptive measures calculated
from the sample are called statistics.
•Statistics vary from sample to sample and
hence are random variables.
•The probability distributions for statistics are
called sampling distributions.
•In repeated sampling, they tell us what values
of the statistics can occur and how often each
value occurs.
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Sampling Distributions
Definition: The sampling distribution of a
statistic is the probability distribution for the
possible values of the statistic that results when
random samples of size n are repeatedly drawn
from the population.
Population: 3, 5, 2, 1
Draw samples of size n = 3
without replacement
p(x)
x
Possible samples
10 / 3  3.33
3, 5, 2
3, 5, 1
9/3  3
3, 2, 1
6/3  2
5, 2, 1
8 / 3  2.67
Each value of
x-bar is
equally
likely, with
probability
1/4
1/4
2
3
x
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Sampling Distributions
Sampling distributions for statistics can be
Approximated with simulation techniques
Derived using mathematical theorems
The Central Limit Theorem is one such
theorem.
Central Limit Theorem: If random samples of n
observations are drawn from a nonnormal population with
finite m and standard deviation s , then, when n is large, the
sampling distribution of the sample mean x is approximately
normally distributed, with mean m and standard deviation
s / n. The approximation becomes more accurate as n
becomes large.
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Example
Applet
Toss a fair coin n = 1 time. The distribution of x the
number on the upper face is flat or uniform.
m   xp( x)
1
1
1
 1( )  2( )  ...  6( )  3.5
6
6
6
s  ( x  m ) 2 p( x)  1.71
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Example
Applet
Toss a fair coin n = 2 time. The distribution of x the
average number on the two upper faces is moundshaped.
Mean : m  3.5
Std Dev :
s/ 2  1.71/ 2  1.21
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Example
Applet
Toss a fair coin n = 3 time. The distribution of x the
average number on the two upper faces is
approximately normal.
Mean : m  3.5
Std Dev :
s/ 3  1.71/ 3  .987
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Why is this Important?
The
Central Limit Theorem also implies that the
sum of n measurements is approximately normal with
mean nm and standard deviation s n .
Many
statistics that are used for statistical inference
are sums or averages of sample measurements.
When
n is large, these statistics will have
approximately normal distributions.
This
will allow us to describe their behavior and
evaluate the reliability of our inferences.
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How Large is Large?
If the sample is normal, then the sampling
distribution of x will also be normal, no matter
what the sample size.
When the sample population is approximately
symmetric, the distribution becomes approximately
normal for relatively small values of n.
When the sample population is skewed, the sample
size must be at least 30 before the sampling
distribution of x becomes approximately normal.
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The Sampling Distribution of the
Sample Mean
A random sample of size n is selected from a
population with mean m and standard deviation s.
The sampling distribution of the sample mean
have mean m and standard deviation s / n .
x
will
If the original population is normal, the sampling
distribution will be normal for any sample size.
If the original population is nonnormal, the sampling
distribution will be normal when n is large.
The standard deviation of x-bar is sometimes called the
STANDARD ERROR (SE).
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Finding Probabilities for
the Sample Mean
If the sampling distribution of x is normal or
approximately normal, standardize or rescale the
interval of interest in terms of
xm
z
s/ n
Find the appropriate area using Table 3.
Example: A random
sample of size n = 16
from a normal
distribution with m = 10
and s = 8.
12  10
P( x  12)  P( z 
)
8 / 16
 P( z  1)  1  .8413  .1587
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Applet
Example
A soda filling machine is supposed to fill cans of
soda with 12 fluid ounces. Suppose that the fills are
actually normally distributed with a mean of 12.1 oz
and a standard deviation of .2 oz. What is the
probability that the average fill for a 6-pack of soda is
less than 12 oz?
P(x  12) 
x  m 12  12.1
P(

)
s / n .2 / 6
P( z  1.22)  .1112
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The Sampling Distribution of
the Sample Proportion
The Central Limit Theorem can be used to
conclude that the binomial random variable x is
approximately normal when n is large, with mean np
and standard deviation .
x
The sample proportion, pˆ  n is simply a rescaling
of the binomial random variable x, dividing it by n.
From the Central Limit Theorem, the sampling
distribution of pˆ will also be approximately
normal, with a rescaled mean and standard deviation.
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The Sampling Distribution of
the Sample Proportion
A random sample of size n is selected from a
binomial population with parameter p.
The sampling distribution of the sample proportion,
x
pˆ 
n
will have mean p and standard deviation
pq
n
If n is large, and p is not too close to zero or one, the
sampling distribution of pˆ will be approximately
normal.
The standard deviation of p-hat is sometimes called
the STANDARD ERROR (SE)Copyright
of p-hat.
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Finding Probabilities for
the Sample Proportion
If the sampling distribution of pˆ is normal or
approximately normal, standardize or rescale the
interval of interest in terms of z  pˆ  p
pq
n
Find the appropriate area using Table 3.
.5  .4
Example: A random
P( pˆ  .5)  P( z 
)
.4(.6)
sample of size n = 100
100
from a binomial
population with p = .4.  P( z  2.04)  1  .9793  .0207
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Example
The soda bottler in the previous example claims
that only 5% of the soda cans are underfilled.
A quality control technician randomly samples 200
cans of soda. What is the probability that more than
10% of the cans are underfilled?
n = 200
S: underfilled can
p = P(S) = .05
q = .95
np = 10 nq = 190
OK to use the normal
approximation
P( pˆ  .10)
.10  .05
 P( z 
)  P( z  3.24)
.05(.95)
200
 1  .9994  .0006
This would be very unusual,
if indeed
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Types of Inference
• Estimation:
– Estimating or predicting the value of the
parameter
– “What is (are) the most likely values of m
or p?”
• Hypothesis Testing:
– Deciding about the value of a parameter
based on some preconceived idea.
– “Did the sample come from a population
with m  5 or p = .2?”
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Types of Inference
• Examples:
– A consumer wants to estimate the average
price of similar homes in her city before
putting her home on the market.
Estimation: Estimate m, the average home price.
–A manufacturer wants to know if a new
type of steel is more resistant to high
temperatures than an old type was.
Hypothesis test: Is the new average resistance, mN
equal to the old average resistance, mO?
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Definitions
• An estimator is a rule, usually a
formula, that tells you how to calculate
the estimate based on the sample.
– Point estimation: A single number is
calculated to estimate the parameter.
– Interval estimation: Two numbers are
calculated to create an interval within
which the parameter is expected to lie.
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Measuring the Goodness
of an Estimator
• The distance between an estimate and the
true value of the parameter is the error of
The distance between the bullet and
estimation.
the bull’s-eye.
• In this chapter, the sample sizes are large,
so that our unbiased estimators will have
normal distributions. Because of the Central
Limit Theorem.
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The Margin of Error
• For unbiased estimators with normal
sampling distributions, 95% of all point
estimates will lie within 1.96 standard
deviations of the parameter of interest.
•Margin of error: The maximum error of
estimation, calculated as
1.96  std error of the estimator
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Estimating Means
and Proportions
•For a quantitative population,
Point estimatorof populationmean μ : x
M argin of error (n  30) :  1.96
s
n
•For a binomial population,
Point estimatorof populationproportionp : pˆ  x/n
pˆ qˆ
Margin of error (n  30) :  1.96
n
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Example
• A homeowner randomly samples 64 homes
similar to her own and finds that the average
selling price is $252,000 with a standard
deviation of $15,000. Estimate the average
selling price for all similar homes in the city.
Point estimatorof μ : x  250,000
s
15,000
Margin of error :  1.96
 1.96
 3675
n
64
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Example
A quality control technician wants to estimate
the proportion of soda cans that are underfilled.
He randomly samples 200 cans of soda and
finds 10 underfilled cans.
n  200
p  proportionof underfilled cans
Point estimatorof p : pˆ  x/n  10 / 200  .05
pˆ qˆ
(.05)(.95)
Margin of error :  1.96
 1.96
 .03
n
200
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Interval Estimation
• Create an interval (a, b) so that you are fairly
sure that the parameter lies between these two
values.
• “Fairly sure” is means “with high probability”,
measured using the confidence coefficient, 1a.
Usually, 1-a  .90, .95, .98, .99
• Suppose 1-a = .95 and
that the estimator has a
normal distribution.
Parameter  1.96SE
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Confidence Intervals
for Means and Proportions
•For a quantitative population,
Confidenceintervalfor a populationmean μ :
s
x  za / 2
n
•For a binomial population,
Confidenceintervalfor a populationproportionp :
pˆ  za / 2
pˆ qˆ
n
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Example
• A random sample of n = 50 males showed a
mean average daily intake of dairy products
equal to 756 grams with a standard deviation of
35 grams. Find a 95% confidence interval for the
population average m.
s
35
x  1.96
 756  1.96
 756  9.70
n
50
or 746.30  m  765.70grams.
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Example
• Find a 99% confidence interval for m, the
population average daily intake of dairy products
for men.
s
35
x  2.58  756  2.58 
n
50
756  12.77
or 743.23  m  768.77 grams.
The interval must be wider to provide for the
increased confidence that is does indeed
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enclose the true value of m.
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Example
• Of a random sample of n = 150 college students,
104 of the students said that they had played on a
soccer team during their K-12 years. Estimate the
porportion of college students who played soccer
in their youth with a 98% confidence interval.
104
.69(.31)
pˆ qˆ

 2.33
pˆ  2.33
150
150
n
or .60  p  .78.
 .69  .09
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Estimating the Difference
between Two Means
•Sometimes we are interested in comparing the
means of two populations.
•The average growth of plants fed using two
different nutrients.
•The average scores for students taught with two
different teaching methods.
•To make this comparison,
A random sample of size n1 drawn from
A random
of size s
n2 2drawn
from
population 1 with
mean μsample
and variance
.
1
1
population 2 with mean μ2 and variance s 22 .
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Estimating the Difference
between Two Means
•We compare the two averages by making
inferences about m1-m2, the difference in the two
population averages.
•If the two population averages are the same,
then m1-m2 = 0.
•The best estimate of m1-m2 is the difference
in the two sample means,
x1  x2
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The Sampling
Distribution of x1  x2
1. The mean of x1  x2 is m1  m 2 , the difference in
the population means.
2. The standard deviation of x1  x2 is SE 
s 12
n1

s 22
n2
.
3. If the sample sizes are large, the sampling distributi on
of x1  x2 is approximat ely normal, and SE can be estimated
as SE 
s12 s22
 .
n1 n2
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Estimating m1-m2
•For large samples, point estimates and their
margin of error as well as confidence intervals
are based on the standard normal (z)
distribution. Point estimate for m1 - m 2 : x1  x2
2
1
2
2
s
s
Margin of Error :  1.96

n1 n2
Confidence interval for m1 - m 2 :
( x1  x2 )  za / 2
s12 s22

n1 n2
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Example
Avg Daily Intakes
Men
Women
Sample size
50
50
Sample mean
756
762
Sample Std Dev
35
30
• Compare the average daily intake of dairy products of
men and women using a 95% confidence interval.
s12 s22
( x1  x2 )  1.96

n1 n2
35 30
 (756  762)  1.96

50 50
  6  12.78
or -18.78  m1  m2  6.78.
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Example, continued
-18.78  m1  m2  6.78
• Could you conclude, based on this confidence
interval, that there is a difference in the average daily
intake of dairy products for men and women?
• The confidence interval contains the value m1-m2= 0.
Therefore, it is possible that m1 = m2. You would not
want to conclude that there is a difference in average
daily intake of dairy products for men and women.
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Estimating the Difference
between Two
•We compare the two proportions by making
inferences about p1-p2, the difference in the two
population proportions.
•If the two population proportions are the
same, then p1-p2 = 0.
•The best estimate of p1-p2 is the difference
in the two sample proportions,
x1 x2
pˆ 1  pˆ 2  
n1 n2
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The Sampling
Distribution of pˆ1  pˆ 2
1. The mean of pˆ 1  pˆ 2 is p1  p2 , the difference in
the population proportion s.
2. The standard deviation of pˆ 1  pˆ 2 is SE 
p1q1 p2 q2

.
n1
n2
3. If the sample sizes are large, the sampling distributi on
of pˆ 1  pˆ 2 is approximat ely normal, and SE can be estimated
as SE 
pˆ 1qˆ1 pˆ 2 qˆ 2

.
n1
n2
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Estimating p1-p2
•For large samples, point estimates and their
margin of error as well as confidence intervals
are based on the standard normal (z)
distribution. Point estimate for p -p : pˆ  pˆ
1 2
1
2
pˆ1qˆ1 pˆ 2 qˆ 2
Margin of Error :  1.96

n1
n2
Confidence interval for p1  p2 :
( pˆ1  pˆ 2 )  za / 2
pˆ1qˆ1 pˆ 2 qˆ 2

n1
n2
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Example
Youth Soccer
Male
Female
Sample size
80
70
Played soccer
65
39
• Compare the proportion of male and female college
students who said that they had played on a soccer team
during their K-12 years using a 99% confidence interval.
pˆ1qˆ1 pˆ 2 qˆ2
( pˆ1  pˆ 2 )  2.58

n1
n2
65 39
.81(.19) .56(.44)
 (  )  2.58

 .25  .19
80 70
80
70
or .06  p1  p2  .44.
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Example, continued
.06  p1  p2  .44
• Could you conclude, based on this confidence
interval, that there is a difference in the proportion of
male and female college students who said that they
had played on a soccer team during their K-12 years?
• The confidence interval does not contains the value
p1-p2 = 0. Therefore, it is not likely that p1= p2. You
would conclude that there is a difference in the
proportions for males and females.
A higher proportion of males than
females played Copyright
soccer©2003
in their
youth.
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One Sided
Confidence Bounds
• Confidence intervals are by their nature
two-sided since they produce upper and
lower bounds for the parameter.
• One-sided bounds can be constructed
simply by using a value of z that puts a
rather than a/2 in the tail of the z
LCB : Estimator  za  (Std Error of Estimator)
distribution.
UCB : Estimator  za  (Std Error of Estimator)
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The Sampling Distribution
of the Sample Mean
• When we take a sample from a normal
x
population, the sample mean
has a
normal distribution for any sample size n,
and
xm
xm
is not normal!
z
s/ n
s/ n
• has a standard normal distribution.
• But if s is unknown, and we must use s to
estimate it, the resulting statistic is not
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Student’s t Distribution
• Fortunately, this statistic does have a
sampling distribution that is well known
to statisticians, called the Student’s t
distribution, with n-1 degrees of
freedom.
xm
t
s/ n
•We can use this distribution to create
estimation testing procedures for the population
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Properties of Student’s t
•Mound-shaped and
symmetric about 0.
•More variable than
z, with “heavier tails”
•
•
Shape depends on the sample size n or
the degrees of freedom, n-1. Applet
As n increases the shapes of the t and z
distributions become almost identical.
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Using the t-Table
•
•
Table 4 gives the values of t that cut off
certain critical values in the tail of the t
distribution.
Index df and the appropriate tail area a to
a random
size n =
find ta,the value of For
t with
areasample
a toofits
10, find a value of t that cuts off
right.
.025 in the right tail.
Row = df = n –1 = 9
Column subscript = a = .025
t.025 = 2.262
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Small Sample Inference
for a Population Mean m
•
The basic procedures are the same as
those used for large samples. For a test
of hypothesis:
T est H 0 : m  m 0 versus H a : one or t wo t ailed
using t he t est st at ist ic
x  m0
t
s/ n
using p - values or a reject ionregion based on
a t - dist ribut ion wit h df  n  1.
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Small Sample Inference
for a Population Mean m
•
For a 100(1a)% confidence interval
for the population mean m:
s
x  ta / 2
n
where ta / 2 is the value of t thatcuts off areaa/2
in thetailof a t - distribution withdf  n  1.
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Example
A sprinkler system is designed so that the
average time for the sprinklers to activate
after being turned on is no more than 15
seconds. A test of 5 systems gave the
following times:
17, 31, 12, 17, 13, 25
Is the system working as specified? Test using
0 : m  15 (working as specified)
a =H
.05.
H a : m  15 (not worki ng as specified)
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Example
Data: 17, 31, 12, 17, 13, 25
First, calculate the sample mean and
standard deviation, using your
calculator or the formulas in Chapter 2.
 xi 115
x

 19.167
n
6
( x)
115
x 
2477 
n 
6  7.387
s
n 1
5
2
2
2
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Example
Data: 17, 31, 12, 17, 13, 25
Calculate the test statistic and find the
rejection region for a =.05.
Test statistic :
Degrees of freedom :
x  m 0 19.167  15
t

 1.38 df  n  1  6  1  5
s / n 7.387 / 6
Rejection Region: Reject H0 if t
> 2.015. If the test statistic falls
in the rejection region, its p-value
will be less than a = .05.
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Conclusion
Data: 17, 31, 12, 17, 13, 25
Compare the observed test statistic to
the rejection region, and draw
Test statistic : t  1.38
conclusions.
H 0 : m  15
Rejection Region :
H a : m  15
Reject H if t  2.015.
0
Conclusion: For our example, t = 1.38 does not fall in the
rejection region and H0 is not rejected. There is insufficient
evidence to indicate that the average activation time is greater
than 15.
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Testing the Difference
between Two Means
Asin Chapter9, independent random samplesof sizen1 and n2 are drawn
from populations1 and 2 with means μ1 and μ2 and variancess 12and s 22 .
Since the samplesizesare small,the twopopulations must be normal.
•To test:
•H0: m1m2  D0 versus Ha: one of three
where D0 is some hypothesized difference,
usually 0.
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Testing the Difference
between Two Means
•The test statistic used in Chapter 9
z
x1  x2
s12 s22

n1 n2
•does not have either a z or a t distribution, and
cannot be used for small-sample inference.
•We need to make one more assumption, that
the population variances, although
unknown, are equal.
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Testing the Difference
between Two Means
•Instead of estimating each population variance
separately, we estimate the common variance
with
2
2
•And
the
resulting
(
n

1
)
s

(
n

1
)
s
1
2
2
s2  1
test statistic,
n1  n2  2
t
x1  x2  D0
1 1
s   
 n1 n2 
2
has a t distribution
with n1+n2-2
degrees of freedom.
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Estimating the Difference
between Two Means
•You can also create a 100(1-a)% confidence
interval for m1-m2.
Remember the three
( x1  x2 )  ta / 2
1 1
s   
 n1 n2 
2
2
2
(
n

1
)
s

(
n

1
)
s
1
2
2
with s 2  1
n1  n2  2
assumptions:
1. Original
populations normal
2. Samples random
and independent
3. Equal population
variances.
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Example
• Two training procedures are compared by
measuring the time that it takes trainees to
assemble a device. A different group of trainees are
taught using each method. Is there a difference in the
two methods? Use a = .01.
Time to
Assemble
Method 1
Method 2
Sample size
10
12
Sample mean
35
31
Sample Std
Dev
4.9
4.5
H 0 : m1  m 2  0
H a : m1  m 2  0
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Applet
Example
• Solve this problem by approximating the pvalue using Time to
Method 1 Method 2
Assemble
Table 4.
Calculate :
Sample size
10
12
Sample mean
35
31
Sample Std
Dev
4.9
4.5
2
2
(
n

1
)
s

(
n

1
)
s
1
2
2
s2  1
n1  n2  2
9(4.9 2 )  11(4.52 )

 21.942
20
Test statistic :
t
35  31
1 1
21.942  
 10 12 
 1.99
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Example
p - value : P(t  1.99)  P(t  1.99)
1
P(t  1.99)  ( p - value)
2
df = n1 + n2 – 2 = 10 + 12 – 2 = 20
.025 < ½( p-value) < .05
.05 < p-value < .10
Since the p-value is
greater than a = .01, H0
is not rejected. There is
insufficient evidence to
indicate a difference in
the population means.
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Testing the Difference
between Two Means
•How can you tell if the equal variance
assumption is reasonable?
Rule of Thumb :
larger s 2
If the ratio,
 3,
2
smaller s
the equal variance assumption is reasonable .
larger s 2
If the ratio,
 3,
2
smaller s
use an alternativ e test statistic.Copyright ©2003 Brooks/Cole
A division of Thomson Learning, Inc.
Testing the Difference
between Two Means
•If the population variances cannot be assumed
equal, the test statistic
t
x1  x2
s12 s22

n1 n2
2
s
s 
  
n1 n2 

df  2
( s1 / n1 ) 2 ( s22 / n2 ) 2

n1  1
n2  1
2
1
2
2
•has an approximate t distribution with degrees
of freedom given above. This is most easily
done by computer.
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The Paired-Difference
Test
•Sometimes the assumption of independent
samples is intentionally violated, resulting in a
matched-pairs or paired-difference test.
•By designing the experiment in this way, we can
eliminate unwanted variability in the experiment
by analyzing only the differences,
di = x1i – x2i
•to see if there is a difference in the two
population means, m1m2.
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Example
Car
1
2
3
4
5
Type A
10.6
9.8
12.3
9.7
8.8
Type B
10.2
9.4
11.8
9.1
8.3
• One Type A and one Type B tire are randomly assigned
to each of the rear wheels of five cars. Compare the
average tire wear for types A and B using a test of
hypothesis.
• But the samples are not
independent. The pairs of
H 0 : m1  m 2  0
responses are linked because
H a : m1  m 2  0
measurements are taken on the
same car.
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The Paired-Difference
Test
T o test H 0 : m1  m 2  0 we test H 0 : m d  0
using the test statistic
d 0
t
sd / n
where n  number of pairs, d and sd are the
mean and standarddeviationof thedifferences, d i .
Use the p - value or a rejectionregion based on
a t - distribution withdf  n  1.
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Example
Car
1
2
3
4
5
Type A
10.6
9.8
12.3
9.7
8.8
Type B
10.2
9.4
11.8
9.1
8.3
Difference
.4
.4
.5
.6
.5
H 0 : m1  m 2  0
H a : m1  m 2  0
Test statistic :
 di
Calculated 
 .48
n

 di 

d 0
.48  0
t

 12.8
sd / n .0837 / 5
2
sd 
d
2
i
n 1
n
 .0837
Copyright ©2003 Brooks/Cole
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Example
Car
1
2
3
4
5
Type A
10.6
9.8
12.3
9.7
8.8
Type B
10.2
9.4
11.8
9.1
8.3
Difference
.4
.4
.5
.6
.5
Rejection region: Reject H0 if
t > 2.776 or t < -2.776.
Conclusion: Since t = 12.8,
H0 is rejected. There is a
difference in the average tire
wear for the two types of tires.
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Inference Concerning
a Population Variance
•Sometimes the primary parameter of interest
is not the population mean m but rather the
population variance s2. We choose a random
sample of size n from a normal distribution.
•The sample variance s2 can be used in its
standardized form:
2
(n  1) s
2
 
s2
• which has a Chi-Square distribution with n - 1
degrees of freedom.
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Inference Concerning
a Population Variance
•Table 5 gives both upper and lower critical
values of the chi-square statistic for a given df.
For example, the value
of chi-square that cuts
off .05 in the upper tail
of the distribution with
df = 5 is 2 =11.07.
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Inference Concerning
a Population Variance
T o test H 0 : s 2  s 02 versus H a : one or t wo tailed
we use the test st at istic
 
2
(n  1) s
s
2
2
0
wit h a reject ionregion based on
a chi - square distribution wit hdf  n  1.
Confidenceinterval:
(n  1) s 2
a / 2
2
s 2 
(n  1) s 2

2
(1a / 2 )
Copyright ©2003 Brooks/Cole
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Example
•A cement manufacturer claims that his cement
has a compressive strength with a standard
deviation of 10 kg/cm2 or less. A sample of n =
10 measurements produced a mean and standard
deviation of 312 and 13.96, respectively.
A test of hypothesis:
H0: s2 = 10 (claim is
correct)
Ha: s2 > 10 (claim is
wrong)
uses the test statistic:
2
2
(
n

1
)
s
9
(
13
.
96
)
2
 

 17.5
2
10
100
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Example
•Do these data produce sufficient evidence to
reject the manufacturer’s claim? Use a = .05.
Rejection region: Reject
H0 if 2  16.919 a  .05.
Conclusion: Since 2=
17.5, H0 is rejected. The
standard deviation of the
cement strengths is more
than 10.
Copyright ©2003 Brooks/Cole
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Inference Concerning
Two Population Variances
•We can make inferences about the ratio of two
population variances in the form a ratio. We
choose two independent random samples of
size n1 and n2 from normal distributions.
•If the two population variances are equal, the
statistic
s12
F 2
s2
•has an F distribution with df1 = n1 - 1 and df2 =
n2 - 1 degrees of freedom.
Copyright ©2003 Brooks/Cole
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Inference Concerning
Two Population Variances
•Table 6 gives only upper critical values of the
F statistic for a given pair of df1 and df2.
For example, the value
of F that cuts off .05 in
the upper tail of the
distribution with df1 = 5
and df2 = 8 is F =3.69.
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Inference Concerning
Two Population Variances
To test H 0 : s  s versusH a : one or two tailed
2
1
2
2
we use the test statistic
s12
F  2 wheres12 is the largerof the two sam plevariances
.
s2
with a rejectionregionbasedon an F distribution with
df1  n1  1 and df 2  n2  1. Confidenceinterval:
2
1
2
2
s
s

 F
s
s
2
2
1
1
df 2 , df1
2
2
df1 , dfCopyright
2
2
2
©2003 Brooks/Cole
s
1
s F
A division of Thomson Learning, Inc.
Example
•An experimenter has performed a lab
experiment using two groups of rats. He wants
to test H0: m1  m2, but first he wants to make
sure that the population variances are equal.
Standard (2)
Experimental (1)
Sample size
10
11
Sample mean
13.64
12.42
Sample Std Dev
2.3
5.8
Preliminary test:
H0 : s 12  s 22 versus Ha : s 12  s 22
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Example
Standard (2) Experimental (1)
Sample size
10
11
Sample Std Dev
2.3
5.8
H0 :s  s
2
1
2
2
H a : s 12  s 22
Test statistic :
2
1
2
2
2
s
5 .8
F 
 6.36
2
s
2.3
We designate the sample with the larger standard
deviation as sample 1, to force the test statistic
into the upper tail of the F distribution.
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Example
H 0 : s 12  s 22
Test statistic :
H a :s  s
s12 5.82
F 2 
 6.36
2
s2 2.3
2
1
2
2
The rejection region is two-tailed, with a = .05, but we only
need to find the upper critical value, which has a/2 = .025 to
its right.
From Table 6, with df1=10 and df2 = 9, we reject H0 if F >
3.96.
CONCLUSION: Reject H0. There is sufficient evidence to
indicate that the variances are unequal. Do not rely on the
assumption of equal variances for your t test!
Copyright ©2003 Brooks/Cole
A division of Thomson Learning, Inc.
Parts of a Statistical Test
1. The null hypothesis, H0:
– Assumed to be true until we can
prove otherwise.
2. The alternative hypothesis, Ha:
– Will be accepted as true if we can
disprove H0
Court trial:
Pharmaceuticals:
H0: innocent
H0: m does not exceeds allowed amount
Ha: guilty
Ha: m exceeds allowed
amount
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Parts of a Statistical Test
3. The test statistic and its p-value:
• A single statistic calculated from the
sample which will allow us to reject or
not reject H0, and
• A probability, calculated from the test
statistic that measures whether the test
statistic is likely or unlikely, assuming H0
is true.
4. The rejection region:
–
A rule that tells us for which values of the
Copyright ©2003 Brooks/Cole
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Learning,
Inc.
test statistic, or for which Ap-values,
null
Parts of a Statistical Test
5. Conclusion:
–
Either “Reject H0” or “Do not reject H0”,
along with a statement about the
reliability of your conclusion.
How do you decide when to reject H0?
–
–
Depends on the significance level, a, the
maximum tolerable risk you want to have
of making a mistake, if you decide to
reject H0.
Usually, the significance level is a  .01
or a  .05.
Copyright ©2003 Brooks/Cole
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Large Sample Test of a
Population Mean, m
• Take a random sample of size n 30
from a population with mean m and
standard deviation s.
• We assume that either
1. s is known or
2. s  s since n is large
• The hypothesis to be tested is
– H0:m = m0 versus Ha: mCopyright
 m0©2003 Brooks/Cole
A division of Thomson Learning, Inc.
Test Statistic
• Assume to begin with that H0 is true.
x
The sample mean
is our best estimate
of m, and we use it in a standardized
form as the test statistic:
x  m0 x  m0
z

s / n s/ n
since x has an approximate normal distribution
with mean m0 and standard deviation s / n .
Copyright ©2003 Brooks/Cole
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Applet
Example
• The daily yield for a chemical plant
has averaged 880 tons for several years.
The quality control manager wants to know
if this average has changed. She randomly
selects 50 days and records an average
yield of 871 tons with a standard deviation
ofH21: mtons.
Test statistic :

880
0
x

m
871

880
0
H a : m  880 z 

 3.03
s/ n
21 / 50
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Applet
Example
What is the probability that this test
statistic or something even more extreme
(far from what is expected if H0 is true)
could have happened just by chance?
p - value : P( z  3.03)  P( z  3.03)
 2 P( z  3.03)  2(.0012)  .0024
This is an unlikely
occurrence, which
happens about 2 times in
1000, assuming m = 880!
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Example
• To make our decision clear, we choose
a significance level, say a = .01.
If the p-value is less than a, H0 is rejected as false. You
report that the results are statistically significant at
level a.
If the p-value is greater than a, H0 is not rejected. You
report that the results are not significant at level a.
Since our p-value =.0024 is less than, we reject
H0 and conclude that the average yield has
changed.
Copyright ©2003 Brooks/Cole
A division of Thomson Learning, Inc.
Using a Rejection
Region
If a = .01, what would be the critical
value that marks the “dividing line” between “not
rejecting” and “rejecting” H0?
If p-value < a, H0 is rejected.
If p-value > a, H0 is not rejected.
The dividing line occurs when p-value = a. This is
called the critical value of the test statistic.
Test statistic > critical value implies p-value < a, H0 is rejected.
Test statistic < critical value implies p-value > a, H0 is not
rejected.
Copyright ©2003 Brooks/Cole
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Applet
Example
What is the critical value of z that
cuts off exactly a/2 = .01/2 = .005 in the tail
of the z distribution?
For our example, z
= -3.03 falls in the
rejection region
and H0 is rejected
at the 1%
significance level.
Rejection Region: Reject H0 if z > 2.58 or z < -2.58. If the
test statistic falls in the rejection region, its p-value will be
less than a = .01.
Copyright ©2003 Brooks/Cole
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One Tailed Tests
• Sometimes we are interested in a
detecting a specific directional difference
in the value of m.
• The alternative hypothesis to be tested is
one tailed:
– Ha:m  m0 or Ha: m < m0
• Rejection regions and p-values are
calculated using only one tail of the
sampling distribution.
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Applet
Example
• A homeowner randomly samples 64 homes
similar to her own and finds that the average
selling price is $252,000 with a standard
deviation of $15,000. Is this sufficient evidence
to conclude that the average selling price is
greater than $250,000? Use a = .01.
H 0 : m  250,000
Test statistic :
x  m 0 252,000  250,000

 1.07
H a : m  250,000 z 
s/ n
15,000 / 64
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Critical Value
Approach
What is the
critical value of z that
cuts off exactly a= .01 in the right-tail of the z
distribution?
For our example, z =
1.07 does not fall in
Applet
the rejection region
and H0 is not rejected.
There is not enough
evidence to indicate
that m is greater than
$250,000.
Rejection Region: Reject H0 if z > 2.33. If the test statistic falls
in the rejection region, its p-value will be less than a = .01.
Copyright ©2003 Brooks/Cole
A division of Thomson Learning, Inc.
p-Value Approach
• The probability that our sample results
or something even more unlikely would
have occurred just by chance, when m =
250,000.
p - value : P( z  1.07)  1  .8577  .1423
Applet
Since the p-value is
greater than a = .01, H0
is not rejected. There is
insufficient evidence to
indicate that m is greater
than $250,000.
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Statistical Significance
• If the p-value is less than .01, reject H0.
The results are highly significant.
• If the p-value is between .01 and .05,
reject H0. The results are statistically
significant.
• If the p-value is between .05 and .10, do
not reject H0. But, the results are tending
towards significance.
• If the p-value is greater than .10, do not
reject H0. The results are not statistically
significant.
Copyright ©2003 Brooks/Cole
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Two Types of Errors
There are two types of errors which can
occur in a statistical test.
Actual Fact Guilty
Jury’s
Decision
Innocent
Actual Fact H0 true
Your
(Accept H0)
Decision
H0 false
(Reject H0)
Guilty
Correct
Error
Correct
Type II Error
Innocent
Error
Correct
H0 true
(Accept H0)
H0 false
(Reject H0)
Type I Error Correct
Define:
a = P(Type I error) = P(reject H0 when H0 is true)
b P(Type II error) = P(accept H0 when H0 is false)
Copyright ©2003 Brooks/Cole
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Testing the Difference
between Two Means
A random sample of size n1 drawn from
population 1 with mean μ1 and variance s .
2
1
A random sample of size n2 drawn from
population 2 with mean μ2 and variance s 22 .
•The hypothesis of interest involves the
difference, m1m2, in the form:
•H0: m1m2  D0 versus Ha: one of three
where D0 is some hypothesized difference,
usually 0.
Copyright ©2003 Brooks/Cole
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The Sampling
Distribution of x1  x2
1. The mean of x1  x2 is m1  m 2 , the difference in
the population means.
2. The standard deviation of x1  x2 is SE 
s 12
n1

s 22
n2
.
3. If the sample sizes are large, the sampling distributi on
of x1  x2 is approximat ely normal, and SE can be estimated
as SE 
s12 s22
 .
n1 n2
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Testing the Difference
between Two Means
H 0 : m1  m 2  D0 versus
H a : one of three alternativ es
Test statistic : z 
x1  x2
s12 s22

n1 n2
with rejection regions and/or p - values
based on the standard normal z distributi on.
Copyright ©2003 Brooks/Cole
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Example
Avg Daily Intakes
Men
Women
Sample size
50
50
Sample mean
756
762
Sample Std Dev
35
30
• Is there a difference in the average daily intakes of dairy
products for men versus women? Use a = .05.
H0 : m1  m2  0 (same) H a : m1  m2  0 (different )
Test statistic :
756  762  0
x1  x2  0

 .92
z
2
2
2
2
35
30
s1 s2


50 50
n1 n2
Copyright ©2003 Brooks/Cole
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p-Value Approach
• The probability of observing values of z
that as far away from z = 0 as we have,
just by chance, if indeed m1m2 = 0.
p - value : P( z  .92)  P( z  .92)
 2(.1788)  .3576
Since the p-value is
greater than a = .05, H0
is not rejected. There is
insufficient evidence to
indicate that men and
women have different
average daily intakes.
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Critical Value Approach
What is the critical value of z that
cuts off exactly a= .05 in the left-tail of
the z distribution?
For our example, z = -1.25
does not fall in the
rejection region and H0 is
not rejected. There is not
enough evidence to
indicate that p is less than
.2 for people over 40.
Rejection Region: Reject H0 if z < -1.645. If the test statistic
falls in the rejection region, its p-value will be less than a = .05.
Copyright ©2003 Brooks/Cole
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Testing the Difference
between Two Proportions
•To compare two binomial proportions,
A random sample of size n1 drawn from
binomial population 1 with parameter p1.
A random sample of size n2 drawn from
binomial population 2 with parameter p2 .
•The hypothesis of interest involves the
difference, p1p2, in the form:
H0: p1p2  D0 versus Ha: one of three
•where D0 is some hypothesized difference,
usually 0.
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The Sampling
Distribution of pˆ1  pˆ 2
1. The mean of pˆ 1  pˆ 2 is p1  p2 , the difference in
the population proportion s.
2. The standard deviation of pˆ 1  pˆ 2 is SE 
p1q1 p2 q2

.
n1
n2
3. If the sample sizes are large, the sampling distributi on
of pˆ 1  pˆ 2 is approximat ely normal.
4. The standard error is estimated differentl y, depending on
the hypothesiz ed difference , D 0 .
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Testing the Difference
between Two Proportions
H 0 : p1  p2  0 versus
H a : one of three alternativ es
pˆ 1  pˆ 2
Test statistic : z 
1 1
pˆ qˆ   
 n1 n2 
with pˆ 
x1  x2
to estimate the common val ue of p
n1  n2
and rejection regions or p - values
based on the standard normal z distributi on.
Copyright ©2003 Brooks/Cole
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Example
Youth Soccer
Male
Female
Sample size
80
70
Played soccer
65
39
• Compare the proportion of male and female college
students who said that they had played on a soccer team
during their K-12 years using a test of hypothesis.
H0 : p1  p2  0 (same)
H a : p1  p2  0 (different )
Calculate pˆ1  65 / 80  .81
pˆ 2  39 / 70  .56
x1  x2 104
pˆ 

 .69
n1  n2 150
Copyright ©2003 Brooks/Cole
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Example
Youth Soccer
Male
Female
Sample size
80
70
Played soccer
65
39
Test statistic :
.81  .56
pˆ 1  pˆ 2  0

 3.30
z
1 
 1
1 1
.69(.31)  
pˆ qˆ   
 80 70 
 n1 n2 
p - value : P( z  3.30)  P( z  3.30)  2(.0005)  .001
Since the p-value is less than a = .01, H0 is rejected. The
results are highly significant. There is evidence to indicate
that the rates of participation are differentCopyright
for boys
and girls.
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