Thousands of high school students will take the SAT this weekend. Many of them will try to boost their scores with tricks learned from high-priced tutoring services, which have turned Americans’ college anxiety into a booming business. The number of test prep centers in the U.S. more than doubled to 11,000 from 1998 to 2012, the last year for which Census data are available.

There’s a multibillion-dollar market for tutoring services in the U.S., with franchises such as Kumon and big chains including Kaplan and Princeton Review. The test prep industry promises to help students score better on everything from the SAT to Advanced Placement courses to med school entrance exams.

Tutoring businesses are more likely to pop up in populous states, where there are dense concentrations of striving students, with families willing to pay for lessons that can exceed $200 an hour. New Jersey had 16 tutoring businesses for every 100,000 residents aged 23 or younger, the highest rate of any state. Wyoming had two tutoring centers per 100,000, the lowest.

Do states with more testing centers do better on the SATs? How effective test prep courses are is an open debate, and looking at Census data on tutoring establishments is a very blunt tool for answering the question. Scoring data from the College Board, which administers the SAT, shows a scattershot relationship between test scores and test prep businesses. Idaho, which has among the fewest tutoring businesses, had the second-lowest SAT scores in the country in 2013. Then again, North Dakota, where tutors are also sparse, had the second-highest scores.

Test prep services only affect the scores of students who can afford to pay for them, meaning they’re likely to have less impact in states with poorer test-takers. In some states, students are more likely to take the ACT than the SAT, introducing another selection bias.

The tutoring business has grown fastest in Georgia, where the number of establishments quintupled from 1998 to 2012. SAT scores over the same period declined slightly. A 2009 paper from the National Association for College Admission Counseling, meanwhile, found that test prep courses help students improve scores—but not by as much (PDF) as test prep companies would have students and parents think.

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