A new national study of urban charter schools shows uneven student growth in math and reading, but Boston and Newark charter schools stand out.

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Friends and foes of charter schools will find support for their views in the latest installment of a massive study comparing more than 1 million charter school students to similar students attending traditional public schools.

On average, the charter school students in the study (in 41 urban areas across 22 states) achieved higher annual growth in math and (to a lesser degree) in reading, according to a new study from the Center for Research on Education Outcomes (CREDO) at Stanford University, though performance varied greatly from city to city.

And whether those differences are large enough to be considered meaningful is a matter of debate among social scientists.

Similar findings in the 2009 and 2013 CREDO reports prompted some researchers to warn against using the studies to make overblown claims either in favor of or against charter schools.

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However, the 2015 CREDO report identified two urban areas — Boston and Newark — as places where charter school students are outpacing their peers in traditional school districts by substantial margins, according to at least one commonly cited rule of thumb for measuring impacts.

The CREDO researchers studied student test data from  2006-07 through 2011-12, comparing each charter school student with up to seven counterparts in traditional school districts based on prior test scores and background characteristics such as family income.

One issue: The CREDO study doesn’t address the possibility that families that take the time and effort to learn about and enroll in a charter school may be more motivated about their child’s education than families with students who are attending their assigned schools.

Still, the CREDO results — at least in Boston — are consistent with other studies that have been able to take advantage of a natural experiment that occurs when charter schools have more applicants than seats.

MIT economist Joshua Angrist and his colleagues, for example, have compared Boston students who are selected in random lotteries for charters schools with students who also wanted to attend, but were not selected.

Earlier this month, Spokane’s first two charter schools, expected to open in September, held lotteries because they had more applicants than seats, which could provide researchers with an opportunity to do a similar type of experiment.