A study found that Harvard freshmen who had not taken SAT preparation courses scored an average of 63 points higher on the SAT than did Harvard freshmen who had taken such courses. Harvard’s admissions director said that this study suggested that SAT preparation courses are ineffective [29].

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

b. Suppose that SAT preparation courses increase SAT scores by 25 points. Explain how it could still be the case that students who do not take such courses score higher, on average, than do students who do take these courses.

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