The Stanford study compared students with very similar backgrounds -- such as baseline test scores, race and income level -- to see how those who went to charters fared compared to those who attended traditional public school in the same district. Charter students were matched against students at the “feeder school” they likely would have attended in the public school system.

Overseeing the overseers
Statewide, the study found that charter students would fall behind, an equivalent to 14 days in a 180-day school year. Researcher Margaret Raymond says charters in some states are a big improvement over public schools. But in Ohio, where rules for sponsors or “authorizers” are lax, the students suffer.

“I think we need to have a greater degree of oversight of charter schools, but I also we need to have some oversight of the overseers," Raymon says. "Authorizing is a place where those high performing states have really been very deeply attentive -- to their authorizers and how they perform. They hold their authorizers accountable.”

Comparing to other states
In a study last year, Stanford found Ohio charter schools ranked low compared to other states. In the new study, some charters performed very well. Community schools in Cleveland outperformed the municipal school district -- especially for poor black students. But there is a wide disparity among charters. And Raymond, who calls herself a “pro-market girl,” says school choice is still too complicated for most parents.

“It’s the only industry/sector where the market mechanism just doesn’t work," Raymond says. "I think it’s not helpful to expect the parents to be the agents of quality assurance throughout the state. I think there are other supports are needed. ”

Charter schools run by large companies tended to perform worse than independent schools. Raymond says it was a revelation to the researchers how much strong school boards are needed to ensure high quality performance at charter school companies.