Does coaching raise SAT scores? Because many students score higher on a second try even without coaching, a study looked at an simple random sample of 4200 students who took the SAT twice. Of these, 500 had taken coaching courses between their two attempts at the SAT. The study compared the average increase in scores for students who were coached with the average increase for students who were not coached.
The SAT study doesn't show that coaching causes a greater increase in SAT scores. One plausible reason is that
4200 students is too few to draw a conclusion.
more students were not coached than were coached. there was no random selection.
students who chose to coach were different (in particular, they were richer) than students who had no coaching.