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Hypothesis Testing for the Mean (Small Samples)
Larson/Farber 4th ed.
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Section 7.3 Objectives
 Find critical values in a t-distribution
 Use the t-test to test a mean μ
 Use technology to find P-values and use them with a t-
test to test a mean μ
Larson/Farber 4th ed.
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Finding Critical Values in a tDistribution
1. Identify the level of significance .
2. Identify the degrees of freedom d.f. = n – 1.
3. Find the critical value(s) using Table 5 in Appendix B
in the row with n – 1 degrees of freedom. If the
hypothesis test is
a. left-tailed, use “One Tail,  ” column with a
negative sign,
b. right-tailed, use “One Tail,  ” column with a
positive sign,
c. two-tailed, use “Two Tails,  ” column with a
negative and a positive sign.
Larson/Farber 4th ed.
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Example: Finding Critical Values for t
Find the critical value t0 for a left-tailed test given
 = 0.05 and n = 21.
Solution:
• The degrees of freedom are
d.f. = n – 1 = 21 – 1 = 20.
• Look at α = 0.05 in the
“One Tail, ” column.
• Because the test is lefttailed, the critical value is
negative.
Larson/Farber 4th ed.
0.05
-1.725 0
t
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Example: Finding Critical Values for t
Find the critical values t0 and -t0 for a two-tailed test
given  = 0.05 and n = 26.
Solution:
• The degrees of freedom are
d.f. = n – 1 = 26 – 1 = 25.
• Look at α = 0.05 in the
“Two Tail, ” column.
• Because the test is twotailed, one critical value is
negative and one is positive.
Larson/Farber 4th ed.
0.025
-2.060 0
0.025
2.060
t
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t-Test for a Mean μ (n < 30, 
Unknown)
t-Test for a Mean
 A statistical test for a population mean.
 The t-test can be used when the population is normal
or nearly normal,  is unknown, and n < 30.
 The test statistic is the sample meanx
 The standardized test statistic is t.
x 
t
s n
 The degrees of freedom are d.f. = n – 1.
Larson/Farber 4th ed.
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Using the t-Test for a Mean μ
(Small Sample)
In Words
In Symbols
1. State the claim mathematically
and verbally. Identify the null
and alternative hypotheses.
State H0 and Ha.
2. Specify the level of significance.
Identify .
3. Identify the degrees of freedom
and sketch the sampling
distribution.
d.f. = n – 1.
4. Determine any critical value(s).
Use Table 5 in
Appendix B.
Larson/Farber 4th ed.
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Using the t-Test for a Mean μ
(Small Sample)
In Words
In Symbols
5. Determine any rejection
region(s).
6. Find the standardized test
statistic.
7. Make a decision to reject or
fail to reject the null
hypothesis.
8. Interpret the decision in the
context of the original claim.
Larson/Farber 4th ed.
x 
t
s n
If t is in the rejection
region, reject H0.
Otherwise, fail to
reject H0.
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Example: Testing μ with a Small
Sample
A used car dealer says that the mean price of a 2005
Honda Pilot LX is at least $23,900. You suspect this claim
is incorrect and find that a random sample of 14 similar
vehicles has a mean price of $23,000 and a standard
deviation of $1113. Is there enough evidence to reject the
dealer’s claim at α = 0.05? Assume the population is
normally distributed. (Adapted from Kelley Blue Book)
Larson/Farber 4th ed.
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Solution: Testing μ with a Small
Sample
•
•
•
•
•
H0: μ ≥ $23,900
Ha: μ < $23,900
α = 0.05
df = 14 – 1 = 13
Rejection Region:
0.05
-1.771
0
t
-3.026
Larson/Farber 4th ed.
• Test Statistic:
t
x 
s
n

23, 000  23,900
1113 14
 3.026
• Decision: Reject H0
At the 0.05 level of
significance, there is
enough evidence to reject
the claim that the mean
price of a 2005 Honda Pilot
LX is at least $23,900
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Example: Testing μ with a Small
Sample
An industrial company claims that the mean pH level of
the water in a nearby river is 6.8. You randomly select 19
water samples and measure the pH of each. The sample
mean and standard deviation are 6.7 and 0.24,
respectively. Is there enough evidence to reject the
company’s claim at α = 0.05? Assume the population is
normally distributed.
Larson/Farber 4th ed.
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Solution: Testing μ with a Small
Sample
•
•
•
•
•
H0: μ = 6.8
Ha: μ ≠ 6.8
α = 0.05
df = 19 – 1 = 18
Rejection Region:
0.025
-2.101
0.025
0
2.101
t
• Test Statistic:
t
x 
s
n

6.7  6.8
0.24 19
 1.816
• Decision:Fail to reject H0
At the 0.05 level of
significance, there is not
enough evidence to reject
the claim that the mean
pH is 6.8.
-1.816
Larson/Farber 4th ed.
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Example: Using P-values with tTests
The American Automobile Association claims that the
mean daily meal cost for a family of four traveling on
vacation in Florida is $118. A random sample of 11 such
families has a mean daily meal cost of $128 with a
standard deviation of $20. Is there enough evidence to
reject the claim at α = 0.10? Assume the population is
normally distributed. (Adapted from American Automobile
Association)
Larson/Farber 4th ed.
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Solution: Using P-values with t-Tests
• H0: μ = $118
• Ha: μ ≠ $118
TI-83/84set
up:
Calculate:
Draw:
• Decision 0.1664 > 0.10
Fail: to reject H0. At the 0.10 level of significance, there
is not enough evidence to reject the claim that the
mean daily meal cost for a family of four traveling on
vacation in Florida is $118.
Larson/Farber 4th ed.
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Section 7.3 Summary
 Found critical values in a t-distribution
 Used the t-test to test a mean μ
 Used technology to find P-values and used them with
a t-test to test a mean μ
Larson/Farber 4th ed.
15