Reviewer finds serious statistical flaws in research on NYC charter schools

Contact: Sean Reardon, (650) 736-8517 (office); (617) 251-4782 (cell); sean.reardon@stanford.edu
Kevin Welner, (303) 492-8370; kevin.welner@colorado.edu
Gary Miron, (269) 599-7965; gary.miron@wmich.edu

BOULDER, Colo. and TEMPE, Ariz. (November 12, 2009) -- A recent report on New York City charter schools found achievement results at the charters to be better than comparison traditional schools. But that report relies on a flawed statistical analysis, according to a new review.

The report is How New York City's Charter Schools Affect Achievement and was written by Caroline Hoxby, Sonali Murarka, and Jenny Kang. When it was released in late September, it was enthusiastically and uncritically embraced by charter advocates as well as media outlets. The Washington Post offered an editorial titled, "Charter Success. Poor children learn. Teachers unions are not pleased." The editorial's first paragraph reads:

"Opponents of charter schools are going to have to come up with a new excuse: They can't claim any longer that these non-traditional public schools don't succeed. A rigorous new study of charter schools in New York City demolishes the argument that charter schools outperform traditional public schools only because they get the 'best students.' This evidence should spur states to change policies that inhibit charter-school growth. It also should cause traditional schools to emulate practices that produce these remarkable results."

The editorial argues throughout that the study provides unquestionable evidence that charters result in improved student achievement. It ends, "Now the facts are in."

The New York Daily News was no less effusive: "It's official. From this day forward, those who battle New York's charter school movement stand conclusively on notice that they are fighting to block thousands of children from getting superior educations."

Because of the declared importance of the new report, we asked Professor Sean Reardon to carefully examine the report's strengths and weaknesses for the Think Tank Review Project and write a review that would help others use the study in a sensible way. Reardon, like the report's lead author Hoxby, is a professor at Stanford University. He is an expert on research methodology.

The Hoxby report estimates the effects on student achievement of attending a New York City charter school rather than a traditional public school. A key finding, repeated in press reports throughout the U.S., compares the cumulative effect of attending a New York City charter school for nine years (from kindergarten through eighth grade) to the magnitude of average test score differences between students in Harlem and the wealthy New York community of Scarsdale. The report estimates this cumulative effect at roughly 66% of the "Scarsdale-Harlem gap" in English and roughly 86% of the gap in math.

In his review, Reardon observes that the report "has the potential to add usefully to the growing body of evidence regarding the effectiveness of charter schools." New York charter schools' use of randomized lotteries to admit students to charter schools offers the possibility that the study of those schools can roughly approximate laboratory conditions.

But Reardon points out that the report's key findings are grounded in an unsound analysis -- an inappropriate set of statistical models -- and that the report's authors never provide crucial information that would allow readers to more thoroughly evaluate "its methods, results, or generalizability."

Reardon's review notes these shortcomings in the report:

• In measuring the effects of charter schooling on students in grades 4 through 12, the study relies on statistical models that include test scores from the previous year, measured after the admission lotteries take place. Yet because of that timing, those scores could be affected by whether students attend a charter school. As a consequence, the statistical models "destroy the benefits of the randomization" that is a strength of the study's design. (The use of a different model makes the results for students in grades K-3 more credible, he notes.)

• The report's claims regarding the cumulative effects of attending a New York City charter school from kindergarten through eighth grade are based on an inappropriate extrapolation.

• It uses a weaker criterion for statistical significance than is conventionally used in social science research (0.05), referring to p-values of roughly 0.15 as "marginally statistically significant".

• The report describes the variation in charter school effects across schools in a way that may distort the true distribution of effects by omitting many ineffective charter schools from the distribution.

Reardon explains that, as a result of the flaws in the report's statistical analysis, the report "likely overstates the effects of New York City charter schools on students' cumulative achievement, though it is not possible -- given the information missing from the report -- to precisely quantify the extent of overestimation." This, as well as the lack of detailed information in the report to assess the extent of that bias, make it impossible for readers to know whether the report's estimated charter school effects are in fact valid.

"Policymakers, educators, and parents should therefore not rely on these estimates until the bias issues have been fully investigated and the analysis has undergone rigorous peer review."

According to Professor Kevin Welner, director of the University of Colorado at Boulder's Education and the Public Interest Center (EPIC): "Readers of this review will understand that, while Hoxby's charter school study is a contribution, it has significant flaws and limitations. Unfortunately, the editorial reaction of otherwise-respectable media outlets trumpeted the New York City findings as the final and faultless word on charter school performance. In fact, the study used inappropriate methods that overstate the performance of the charter schools it studied."

Welner notes that the Think Tank Review Project also recently reviewed another charter school study, released in June by Stanford's CREDO policy center. That study encompassed 65-70% of the nation's charter schools. "Our review pointed out a number of limitations but also noted the relative strength and comprehensiveness of the data set, the solid analytic approaches of the CREDO researchers, and the important fact that the CREDO results were consistent with a large body of research showing charter schools overall to be performing no better than (and perhaps worse than) traditional public schools," Welner says. But he added that "the CREDO and Hoxby reports used different designs and covered different schools. They are not directly comparable, nor are we able to say which is 'better.' Neither report is definitive or without notable weaknesses."

Welner concludes, "the important thing to understand is that if, after an appropriate reanalysis of the data, we still find that New York City's charter schools are in fact bucking the national trend, the sensible next step is for researchers to explore the causes rather than to jump to broad conclusions that fly in the face of the overall research base. It would be irresponsible to use the NYC results -- even if they were valid and reliable -- to drive policy in places throughout the U.S. where charters are apparently underperforming their competition."

Find Sean Reardon's review on the web at:
http://epicpolicy.org/thinktank/review-How-New-York-City-Charter

Find the NYC report by Hoxby and her colleagues at:
http://www.nber.org/~schools/charterschoolseval/

CONTACT:
Sean F. Reardon
Associate Professor of Education and (by courtesy) Sociology
Stanford University
(650) 736-8517 (office); (617) 251-4782 (cell)
sean.reardon@stanford.edu

Kevin Welner, Professor and Director
Education and the Public Interest Center
University of Colorado at Boulder
(303) 492-8370
kevin.welner@colorado.edu

Gary Miron, Professor of Education
Western Michigan University
(269) 599-7965
gary.miron@wmich.edu

About the Think Tank Review Project

The Think Tank Review Project (http://thinktankreview.org), a collaborative project of the University of Colorado at Boulder Education and the Public Interest Center (EPIC) and the ASU Education Policy Research Unit (EPRU), provides the public, policy makers, and the press with timely, academically sound reviews of selected think tank publications. The project is made possible by funding from the Great Lakes Center for Education Research and Practice.

EPIC and EPRU collaborate to produce policy briefs in addition to think tank reviews. Our goal is to promote well-informed democratic deliberation about education policy by providing academic as well as non-academic audiences with useful information and high quality analyses.

Visit EPIC and EPRU at http://www.educationanalysis.org/

EPIC and EPRU are members of the Education Policy Alliance
(http://educationpolicyalliance.org).

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