15. A psychology professor believes that teaching students the active recall method influences students' grades at the end of the course. He takes a sample of 20 students and teaches them this method and how to apply it. The average score for this sample is M = 568. The population of students typically has a normal distribution with 400 and = 100.

a) Can the professor conclude that students who apply this method obtain significantly better results than the general population? Use a one-sided test with .01.

b) Calculate Cohen's d to estimate the effect size.

c) Write a sentence demonstrating how the results of hypothesis testing and effect size measurement would appear in a research report.

16. McAllister et al. (2012) compared college football and hockey players with college athletes playing non-contact sports to determine if exposure to head impacts over the course of a season had an effect on cognitive performance. In the study, tests of new learning performance were significantly worse for athletes playing contact sports than for those playing non-contact sports. The following table presents data similar to the results obtained in the study.

Athltes (Sport sans contact)	Athltes (Sport avec contact)
11	8
9	5
8	10
10	4
14	8
8	7
7	11
13	3

a) Choose your hypothesis test, do your calculations and answer this question: Are the results of athletes playing contact sports significantly lower than those of athletes playing non-contact sports? (Use a one-tailed test with an alpha of 0.05.)

b) Calculate the value of r2 (the percentage of variance explained by these data) and give its interpretation.

c) Suppose that your samples are not equal, and that in the list corresponding to athletes with contact, you are missing 2 values. What approach can you use to work around this problem?