The Center for Research on Education Outcomes (CREDO) at Stanford University analyzed differences in student performance at charter schools and traditional public schools across 27 states and New York City. The study finds a small positive effect of being in a charter school on reading scores and no impact on math scores; it presents these results as showing a relative improvement in average charter school quality since CREDO’s 2009 study. However, there are significant reasons for caution in interpreting the results. Some concerns are technical: the statistical technique used to compare charter students with “virtual twins” in traditional public schools remains insufficiently justified, and may not adequately control for “selection effects” (i.e., families selecting a charter school may be very different from those who do not). The estimation of “growth” (expressed in “days of learning”) is also insufficiently justified, and the regression models fail to correct for two important violations of statistical assumptions. However, even setting aside all concerns with the analytic methods, the study overall shows that less than one hundredth of one percent of the variation in test performance is explainable by charter school enrollment. With a very large sample size, nearly any effect will be statistically significant, but in practical terms these effects are so small as to be regarded, without hyperbole, as trivial.
Center for Research on Education Outcomes