Derek Briggs, the chairman of the research and evaluation methodology program in the school of education at the University of Colorado at Boulder, has examined how much SAT score increases can be attributed to the effect of test prep.
Test-prep programs generally include three elements: a review of test content, practice on test questions, and orientation to the format of the test. In 2009, in cooperation with NACAC, Mr. Briggs reviewed three national data sets and found the average effect of commercial coaching is positive, but slight. Test-score bumps were more in the neighborhood of 30 points (on a 1,600-point scale at the time), far from what some in the industry claim. He does point out that there may be specific programs that are more effective than others, but evidence to support that is weak.
Considering the results of Mr. Briggs’ and earlier studies, NACAC concludes that test-prep activities and coaching have a “minimal positive effect on both the SAT and the ACT.”
FULL DISCLOSURE: Dr. Briggs is on the faculty at the U. of Colorado at Boulder, where I am a Ph.D. candidate. I have studied with Dr. Briggs in the past.