When Statistical Significance Hides More Than it Reveals
by Jeanne M. Powers & Gene V. Glass - July 02, 2014
Background & Purpose: The What Works Clearinghouse (WWC) recently released a summary of five CREDO studies of charter school outcomes produced between 2009 and 2013. We compared the WWC’s summary, which highlighted the statistical significance of the findings, to the effect sizes reported in the individual reports. We also addressed the findings reported in the original studies that were not summarized in the WWC reports.
Research Design: Analytic essay highlighting the gaps between the findings reported in the summary WWC report, the individual WWC reports for each study, and the original CREDO studies.
Findings: We argue that focusing on statistical significance is potentially misleading. The WCC summary invites the reader to conclude that charter schools had a greater effect on students’ achievement gains than traditional public schools. Comparing across the studies’ effect sizes suggests that the average effect of charter schools on students’ achievement gains is negligible. The WWC reports also do not address the considerable variation in achievement gains within and across subgroups of students and schools.
Conclusion: Summaries generated from research studies should provide an accounting of findings that allows practitioners to assess their practical importance. When these and similar reports are hard to understand and misleading, they run the risk of eroding practitioners’ trust in research and increasing rather than bridging the gulf between research and practice.
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