The report examines whether increasing competition from charter schools has a causal effect on the achievement of public school students in New York City, using a three-year longitudinal database of student test scores. As a measure of competition, it considers the percentage of students who left a public school for a charter school in the prior year. The statistical analysis suggests that increasing competition has no statistically significant impact on math test scores, but that it has small positive effects on language scores. The report does not conclusively demonstrate that the results are explained by increasing competitive pressure on public school administrators; they may also be explained by shifting peer quality or declining short-run class sizes in public schools.