10.23 The independent-samples t test, hypnosis, and the Stroop effect: Using data from Exercise 9.61 on the effects of posthypnotic suggestion on the Stroop effect (Raz et al., 2005), let’s conduct an independent-samples t test. For this test, we will pretend that two sets of people participated in the study, a between-groups design, whereas previously we considered data from a within-groups design. The first score for each original participant will be in the first sample—those not receiving a posthypnotic suggestion. The second score for each original participant will be in the second sample—those receiving a posthypnotic suggestion. Sample 1: 12.6, 13.8, 11.6, 12.2, 12.1, 13.0 Sample 2: 8.5, 9.6, 10.0, 9.2, 8.9, 10.8

a. Conduct all six steps of an independent-samples t test. Be sure to label all six steps.

b. Report the statistics as you would in a journal article.

c. What happens to the test statistic when you switch from having all participants be in both samples to having two separate samples? Given the same numbers, is it easier to reject the null hypothesis with a within-groups design or with a between-groups design?

d. In your own words, why do you think it is easier to reject the null hypothesis in one of these situations than in the other?

e. Calculate the 95% confidence interval.

f. State in your own words what we learn from this confidence interval. g. What information does the confidence interval give us that we also get from the hypothesis test? h. What additional information does the confidence interval give us that we do not get from the hypothesis test? i. Calculate the appropriate measure of effect size.

j. Based on Cohen’s conventions, is this a small, medium, or large effect size? k. Why is it useful to have this information in addition to the results of a hypothesis test?