Statistics for the Behavioral Sciences by Frederick J. Gravetter, Larry B. Wallnau ISBN 10: 1305504917 ISBN 13: 9781305504912

288 CHAPTER 9 | Introduction to the t Statistic
LEARNING CHECK
1. A researcher uses a sample of n = 10 to evaluate a treatment effect and obtains
t = 3.00. If effect size is measured using the percentage of variance accounted for,
then what value will be obtained for r 2 ?
9
a. 18 = 0.50
9
b. 17 = 0.53
3
c. 12 = 0.25
3
d. 11 = 0.27
2. What happens to measures of effect size such as r 2 and Cohen’s d as sample size
increases?
a. They also tend to increase.
b. They tend to decrease.
c. Sample size does not have any great influence on measures of effect size.
d. The effect of sample size depends on other factors such as sample variance.
3. Which set of factors would produce the narrowest confidence interval for estimating
a population mean?
a. A large sample and a large percentage of confidence
b. A large sample and a small percentage of confidence
c. A small sample and a large percentage of confidence
d. A small sample and a small percentage of confidence
4. The results of a hypothesis test are reported as follows: “t(20) = 2.70, p , .05.”
Based on this report, how many individuals were in the sample?
a. 19
b. 20
c. 21
d. cannot be determined from the information provided
ANSWERS
1. A, 2. C, 3. B, 4. C
9.4 Directional Hypotheses and One-Tailed Tests
LEARNING OBJECTIVE
9. Conduct a directional (one-tailed) hypothesis test using the t statistic.
As noted in Chapter 8, the nondirectional (two-tailed) test is more commonly used than
the directional (one-tailed) alternative. On the other hand, a directional test may be used in
some research situations, such as exploratory investigations or pilot studies or when there
is a priori justification (for example, a theory or previous findings). The following example
demonstrates a directional hypothesis test with a t statistic, using the same experimental
situation presented in Example 9.2.
EXAMPLE 9.6
The research question is whether attractiveness affects the behavior of infants looking
at photographs of women’s faces. The researcher is expecting the infants to prefer the