A study found that the grade point averages (GPAs) of women at all-women colleges tend to be higher than the GPAs of women at coeducational colleges, and concluded that women do better when they attend all-women colleges.

a. Explain why there might be sampling bias in this study.

b. Suppose that every woman’s GPA does not depend on the college she attends.

Explain how it still might be true that women at all-women colleges tend to have higher GPAs than do women at coeducational colleges.

c. What would have to be done to have a controlled experiment?