"How New York City's Charter Schools Affect Achievement" estimates the effects on student achievement of attending a New York City charter school rather than a traditional public school and investigates the characteristics of charter schools associated with the most positive effects on achievement. Because the report relies on an inappropriate set of statistical models to analyze the data, however, the results presented appear to overstate the cumulative effect of attending a charter school. In addition, the report does not provide enough technical discussion and detailed description to enable a reader to assess the validity of some aspects of the report's methodology and results. Policymakers, educators, and parents, therefore, should not rely on these estimates until the authors provide more technical detail and the analysis has undergone rigorous peer review. (Contains 21 notes.)