More about confidence intervals, effect sizes, and tennis serves: Let’s assume the average speed of a serve in women’s tennis is around 118 mph, with a standard deviation of 12 mph. We recruit 100 amateur tennis players to use our new training method this time, and after 6 months we calculate a group mean of 123 mph.

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

2. Compute the effect size and describe its strength.