Both groups of Milwaukee schools did better than a control group of schools outside Milwaukee, where no students were eligible for vouchers, the study says. But another researcher who has argued that vouchers are ineffective questioned Hoxby's methods. Martin Carnoy, a professor of education and economics at Stanford University, said school choice is not the only possible explanation for the improvements Hoxby found. As the choice program has grown, Milwaukee Public Schools has taken a newly competitive posture. The district has been running TV ads, and schools have tried everything from open houses to chili dinners to attract students.
Officials are fond of saying that they operate in a free-market education economy.
Hoxby's study, due to be published in a book on school choice, examines the voucher program in Milwaukee and charter schools in Michigan and Arizona. That growth allows comparison of public schools before and after they faced significant voucher competition, Hoxby writes. Hoxby evaluated schools' productivity, which she defined as the ratio of standardized test scores in various subjects to per-student spending. In other words, she considered higher scores to indicate improvement only if they didn't accompany vastly higher spending. Most students receiving vouchers are in elementary school, and Wisconsin gives statewide tests in fourth, eighth and 10th grades, so Hoxby studied fourth-grade scores.