The nation's charter school students showed more academic gains in reading than their public school counterparts did according to a new study by the Center for Research on Education Outcomes at Stanford University. The study showed no difference in math performance between the two groups.

California charter school students performed better in reading but worse in math than a comparable public school.

National Charter School Study 2013 is an update to the center's 2009 report of charter school performance, which was less flattering.

In both studies, the center said it compared the performance of students  at charter schools with a “virtual twin” counterpart of each student in traditional public schools. The study attempts to show how the charter school students would have done had they attended their local public schools.

In the 2009 study, which looked at performance from 2000 - 2008, the charter school students showed lower academic gains in both reading and math than their "twins."

The improvements since then vary widely according to the types of student analyzed, with higher gains seen in minority and poorer students, according to the study's authors. The results also varied greatly state-by-state.

“It’s pretty apparent to us that charter schools have a strong and positive effect on students who are black, students who are in poverty, students who are [English language learners] and particularly students who are minority and poverty,” said Margaret Raymond, the lead researcher on the study. “Those are the places where the charter school impact seems to be the largest.”

Since the 2009 study, some low-performing charter schools studied closed and some high-performing ones were added to the study group.

The study utilized records from more than 1.5 million charter school students in 25 states, the District of Columbia and New York City. The data came from a period running from 2006 - 2011. 

The researchers translated the academic gains into days of learning. Overall, charter school students were found to be eight days ahead in reading than public school counterparts. 

While the study didn’t rank the states, charter school students in Rhode Island showed the biggest improvement over public schools with 86 days gained in reading and 108 in math. Those in Nevada fared the worst in comparison to public school “twins”, with 115 days lost in reading and 137 lost in math.

The center said major foundations helped pay for the study, but would not disclose the exact source of its funding because contributors want to remain anonymous.  Researchers retained control of the study and its findings.