The conservative school-choice advocacy organization Thomas B. Fordham Institute published a report claiming that the existence of charters raises test scores in surrounding public schools. The claim is that “a rising tide lifts all boats.” This seems counterintuitive when you think about Detroit and Milwaukee, which are flooded with charters but mired at the bottom of urban districts tested by NAEP.

Now this assertion has been reviewed by a scholar and found to lack validity.

William J. Mathis: (802) 383-0058,
Yongmei Ni: (801) 587-9298,

Report About Charters Being a “Rising Tide” Sinks Under Weight of Flawed Data

An NEPC Review funded by the Great Lakes Center

Key Takeaway: Due to data and methods limitations, report fails to prove its claim that higher charter market share is associated with achievement gains for all students.

EAST LANSING, MI (November 14, 2019) – A recent report from the Thomas B. Fordham Institute examines whether average achievement in a school district increases as the “market share” of charter schools rises. The report argues that there is a positive competition effect.

Yongmei Ni of the University of Utah reviewed Rising Tide: Charter School Market Share and Student Achievement, and determined that its findings have limited use in guiding policy and practice, because of the flawed data and methods it employs.

Using a national data set of school districts with longitudinal records (allowing an analysis of each school district’s changes over time), the report found that overall, higher charter market share is associated with statistically significant increases in average reading achievement (but not math achievement). Further, the report finds some positive relationships 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.”

Professor Ni explains that these findings and conclusions should be interpreted with extreme caution because of major weaknesses 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 those districts with over 95th percentile of charter market share.

Overall, she concludes, 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. For example, it does not examine possible policy factors that might be associated with charter market share in a given area having a positive or negative association with public school systems. Similarly, it does not consider which practices might benefit charter schools and/or public school systems as a whole.

Find the review, by Yongmei Ni, at:

Find Rising Tide: Charter School Market Share and Student Achievement, written by David Griffith and published by the Fordham Institute, at:

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