Charter School Expansion and Achievement: Evidence from Harlem, 1999-2025

We estimate community-wide effects on student achievement, as measured by performance on New York State proficiency tests, of charter school expansions in Harlem. Synthetic difference-in-differences (SDID) models estimate trends before and after enactment of the 2007 New York Charter Schools Act amendment, an exogenous supply shock that doubled the limit on the number of charter schools throughout the state, including an additional 50 in New York City. Approximately 400 weighted control schools with near identical pre-treatment trends in test scores are selected from 50 large urban districts in the state. Harlem’s charter enrollment share increased from 11% in 2007 to 47% in 2019, as compared to minimal change in the comparison districts. Post-enactment, Harlem’s ELA and Math proficiency rates, relative to those at the synthetic counterfactual, each rose by 13.8 percentage points. Both charter and district sectors contributed to these gains, though the largest gains came from the charter sector.