As D.C.’s racial and socioeconomic composition has changed, so too have the populations of its traditional and public charter schools. But these kinds of changes alone don’t account for a rise in standardized-test scores over the past decade, a new analysis by the Urban Institute concludes.

The organization examined scores from the National Assessment of Educational Progress, which tests fourth and eighth graders in math and reading, from 2005 to 2013. It then compared them “with the increase that might have been expected based on” shifts in racial, gender, age, and home-language characteristics. The authors report that demographic changes predicted score improvements of between four and six points from 2005 to 2013—a few times less than what actual NAEP scores bore out. In eighth-grade math, for instance, demographics shifts only projected a four point increase across traditional and charter schools; in reality, eighth-grade math saw a 20 point rise. The exception to the findings was eighth-grade reading, where such changes predict “more than half of the increase.”

“D.C. education saw many changes over this period, including reform-oriented chancellors, mayoral control, and a rapidly expanding charter sector, but we cannot identify which policy changes, if any, produced these results,” the group explains. “And despite the large gains, D.C. NAEP scores still reveal substantial achievement gaps—for example, the gap between average scores for black and white students was 56 points in 2015; the gap between Hispanics and whites was 49 points.”

The Urban Institute today launched a tool that adjusts NAEP scores to “account for demographic differences across students in each state.” D.C., which has roughly 87,000 students across its traditional and charter schools, was not included as it’s not a state. Read the analysis here.