As in the previous exercise, assume the average speed of a serve in women’s tennis is around 118 mph, with a standard deviation of 12 mph. But now we recruit only 26 amateur tennis players to use our method. Again, after 6 months we calculate a group mean of 123 mph.

a. Using a 95% confidence interval, test the hypothesis that our method makes a difference.

b. Compute the effect size and describe its strength.

c. How did changing the sample size from 100 (in Exercise 8.54) to 26 affect the confidence interval and effect size? Explain your answer.