The Thomas B. Fordham Institute published a report, A Rising Tide: Charter School Market Share and Student Achievement, examining the relationship between average achievement in a school district and the “market share” of charter schools. Using longitudinal national district-level data, the study finds that overall higher charter market share is associated with significant increases in average reading achievement but not math achievement. Further, some positive relationships exist for specific racial subgroups in districts of certain sizes and geographic locations.  The report concludes that charter schools are “a rising tide” that “lifts all education boats.” However, one should interpret the findings and conclusions with extreme caution because of major issues surrounding the data and methods, including the measure of charter market share, the sample selection criteria, and the overreliance on results based on a small number of districts, especially the ones with over 95th percentile of charter market share. Overall, the findings have little use to policymakers because of these issues with data and methods and because the report does not probe beneath the surface.