Martin Carnoy is a professor at Stanford University who has studied education systems around the world.

Carnoy wrote a report for the Economic Policy Institute about the efficacy of vouchers, or their lack thereof. The report is titled “School Vouchers Are Not a Proven Strategy for Improving Student Achievement.” Carnoy reviews the longest-running voucher programs in the U.S. and other countries and finds little evidence that they improve student achievement.

Here is his summary:

“This report seeks to inform that debate by summarizing the evidence base on vouchers. Studies of voucher programs in several U.S. cities, the states of Florida, Indiana, Louisiana, and in Chile and India, find limited improvements at best in student achievement and school district performance from even large-scale programs. In the few cases in which test scores increased, other factors, namely increased public accountability, not private school competition, seem to be more likely drivers. And high rates of attrition from private schools among voucher users in several studies raises concerns. The second largest and longest-standing U.S. voucher program, in Milwaukee, offers no solid evidence of student gains in either private or public schools.

“In the only area in which there is evidence of small improvements in voucher schools—in high school graduation and college enrollment rates—there are no data to show whether the gains are the result of schools shedding lower-performing students or engaging in positive practices. Also, high school graduation rates have risen sharply in public schools across the board in the last 10 years, with those increases much larger than the small effect estimated on graduation rates from attending a voucher school.

“The lack of evidence that vouchers significantly improve student achievement (test scores), coupled with the evidence of a modest, at best, impact on educational attainment (graduation rates), suggests that an ideological preference for education markets over equity and public accountability is what is driving the push to expand voucher programs. Ideology is not a compelling enough reason to switch to vouchers, given the risks. These risks include increased school segregation; the loss of a common, secular educational experience; and the possibility that the flow of inexperienced young teachers filling the lower-paying jobs in private schools will dry up once the security and benefits offered to more experienced teachers in public schools disappear.

“The report suggests that giving every parent and student a great “choice” of educational offerings is better accomplished by supporting and strengthening neighborhood public schools with a menu of proven policies, from early childhood education to after-school and summer programs to improved teacher pre-service training to improved student health and nutrition programs. All of these yield much higher returns than the minor, if any, gains that have been estimated for voucher students.”

Carnoy published a shorter version of the report for a popular audience. He wrote an article for the New York Daily News explaining why Trump and DeVos are wrong about school choice, specifically vouchers.

He reviews recent research in plain language. Kids don’t benefit. In some places, they actually lose ground.

As I have often written in this space, if vouchers, charters, and school choice were the solution to the problems of urban education, Milwaukee would be the model district of the nation, as it has had choice since 1990. That’s two full generations of students.

He writes:

If the President and his new secretary of education, Betsy DeVos, were right about choice, Milwaukee would be among the highest-scoring urban school districts in the nation. Milwaukee’s private students would be outscoring those in public schools, and students in public schools would have made large gains because of the intense competition from private and charter schools.

None of that is the case. Research over a four-year period that compared the gains of voucher and public school students in Milwaukee showed that the voucher students did no better. And it’s African Americans, who make up roughly two-thirds of Milwaukee’s student body, who are the main recipients of vouchers and also most likely to attend charter schools.

When we compare the National Assessment of Educational Progress scores — that’s the gold standard of achievement tests — of black students in eighth-grade math and reading in 13 urban U.S. school districts, black students in Milwaukee have lower eighth-grade math scores than students in every city but Detroit — notably, another urban district with a high level of school choice.

In reading, Milwaukee’s black eighth-graders do even more poorly. They score lower than black eighth-graders in all other 12 city school districts.

How many billions will we waste on this failed free-market ideology? As Carnoy points out, investing in proven strategies in public schools with credentialed teachers would have long-term benefits.