EVIDENCE MEANS THINGS:

The first results came in late 2015. Researchers examined an Indiana voucher program that had quickly grown to serve tens of thousands of students under Mike Pence, then the state's governor. "In mathematics," they found, "voucher students who transfer to private schools experienced significant losses in achievement." They also saw no improvement in reading.

The next results came a few months later, in February, when researchers published a major study of Louisiana's voucher program. Students in the program were predominantly black and from low-income families, and they came from public schools that had received poor ratings from the state department of education, based on test scores. For private schools receiving more applicants than they could enroll, the law required that they admit students via lottery, which allowed the researchers to compare lottery winners with those who stayed in public school.

They found large negative results in both reading and math. Public elementary school students who started at the 50th percentile in math and then used a voucher to transfer to a private school dropped to the 26th percentile in a single year. Results were somewhat better in the second year, but were still well below the starting point.

This is very unusual. When people try to improve education, sometimes they succeed and sometimes they fail. The successes usually register as modest improvements, while the failures generally have no effect at all. It's rare to see efforts to improve test scores having the opposite result. Martin West, a professor at the Harvard Graduate School of Education, calls the negative effects in Louisiana "as large as any I've seen in the literature" -- not just compared with other voucher studies, but in the history of American education research.

There's always the chance that a single study, no matter how well designed, is an outlier. Studies of older voucher programs in Milwaukee and elsewhere have generally produced mixed results, sometimes finding modest improvements in test scores, but only for some subjects and student groups. Until about a year ago, however, few if any studies had shown vouchers causing test scores to decline drastically.

In June, a third voucher study was released by the Thomas B. Fordham Institute, a conservative think tank and proponent of school choice. The study, which was financed by the pro-voucher Walton Family Foundation, focused on a large voucher program in Ohio. "Students who use vouchers to attend private schools have fared worse academically compared to their closely matched peers attending public schools," the researchers found. Once again, results were worse in math.

Three consecutive reports, each studying one of the largest new state voucher programs, found that vouchers hurt student learning. 

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