Exercises 21, 35, and 37 describe an experiment investigating the effectiveness of omega-3 fats in treating bipolar disorder. Suppose some of the 30 subjects were very active people who walked a lot or got vigorous exercise several times a week, while others tended to be more sedentary, working office jobs and watching a lot of TV. Why might researchers choose to block the subjects by activity level before randomly assigning them to the omega-3 and placebo groups?

Data From Exercise 21

Over a 4-month period, among 30 people with bipolar disorder,  patients who were given a high dose (10 g/day) of omega-3 fats from fish oil improved more than those given a placebo.