A new study of Michigan charter public schools from Stanford University — the most rigorous one to date — found overwhelming positive academic results for charter school students. Measuring average growth in reading scores, 35 percent of charters did better than conventional public schools, while only 2 percent did worse. In math, 42 percent of charters did better and only 6 percent did worse. These overall findings are impressive, but also promising is the study’s evidence suggesting that the charter school market is working too.

Researchers analyzed gains made by charter school students compared to their conventional school peers after controlling for a host of demographic variables. When these results were broken down by year, it showed that the years with the best combined results were the most recent ones — 2010 and 2011. The study’s authors suggest that this could due to a “highly dynamic market” that created a “net positive stock of charter schools.”

From 2005 to 2010, 55 charter schools closed and 94 new ones opened. The fact that the results were higher in the later period suggests that relatively lower-performing charter schools closed and relatively better-performing ones replaced them.

Additionally, a lot has been made of the fact that a large portion (an estimated 75 percent) of charter schools in Michigan are managed by for-profit education management organizations. Some people find this troubling for one reason or another (even though a bunch of other companies and individuals have profited from public schools for decades).

Stanford’s new study suggests that these for-profit EMOs shouldn’t be of much concern. It found that students in charter schools run by education management organizations are achieving annual gains equal to about two and a half months of learning in reading and more than three months in math. Schools operated by EMOs produced better results for students than those not run by EMOs and better results than the overall state average.

A fundamental difference between charter schools and conventional ones is that every student enrolled in a charter is there because of a conscientious decision made by a parent. But parents might choose a charter based on factors other than academic performance, such as school safety, the school’s mission or shared values. The Stanford study’s findings are encouraging, because they suggest that parents are finding and choosing charters where students are making greater learning gains on average, even if those charters are managed by for-profit companies.