From the Fordham report:
To address these questions, Fordham's associate director of research, David Griffith, analyzed a decade of data on reading and math achievement at the metropolitan level, as well as nearly two decades of data on charter and traditional public school enrollment. The results are summarized in three findings:
1. On average, an increase in total charter school enrollment share is associated with a significant increase in the average math achievement of poor, Black, and Hispanic students, which is concentrated in larger metro areas.
2. On average, increases in Black and Hispanic charter school enrollment share are associated with sizable increases in the average math achievement of these student groups, especially in larger metro areas.
3. On average, an increase in total charter school enrollment share is associated with a significant narrowing of a metro's racial and socioeconomic math achievement gaps.