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NOVEMBER 21, 2023

Report Exploring Impact of Smaller Class Size on Student Achievement is Too Limited to Provide Strong Guidance

An NEPC Review funded by the Great Lakes Center

Key Takeaway: By using only a small fragment of available research on the effects of class size, the report significantly limits its usefulness.

GRAND RAPIDS, MI (November 21, 2023) - A 2018 report from Campbell Collaboration, analyzing research literature estimating the impact of class size reduction on reading and math achievement, has recently been gaining renewed attention, thanks in part to a Washington Post piece by Michael Bloomberg. But the body of evidence the report uses is far too limited for the report to be of use to policymakers.

So concludes Clive Belfield of Queens College, City University of New York, in his review of Small Class Sizes for Improving Student Achievement in Primary and Secondary Schools: A Systematic Review.

To reach its findings, the Campbell report conducted an international survey of all research evidence up to 2017-identifying 148 relevant studies. It then narrowed this down to 10 studies meeting its methodological standards. A weighted analysis of the 10 studies-seven of which are from the U.S. and four of which are studies of Tennessee’s STAR-produced estimates of how smaller class size affects achievement. The report found a small positive, statistically significant impact on reading achievement and a small negative, but statistically insignificant, impact on math achievement. Beyond these 10 studies, the report provides a comprehensive catalog of research on class size reduction, and it provides a thorough assessment of each study’s methods.

Professor Belfield contends, however, that the methodology uses an extremely narrow view of what research is valid for determining the impact of reducing class size. More than 90% of studies relevant to the research question were not included. In addition, while related research has appeared in many countries, only three countries are represented in the body of evidence. The result is a very limited analysis of an already narrowed question.

Finally, Professor Belfield concludes, the report's repeated assertions that reducing class size is "costly" are unexplained and unsupported, potentially misleading policymakers about the reform’s potential. These limitations mean the study offers policymakers very little useful guidance for policy or practice. 

Find the review, by Clive Belfield, at:

Find Small Class Sizes for Improving Student Achievement in Primary and Secondary Schools: A Systematic Review, written by Trine Filges, Christoffer Scavenius Sonne-Schmidt, and Bjørn Christian Viinholt Nielsen, and published by the Campbell Collaboration, at:

NEPC Reviews ( provide the public, policymakers, and the press with timely, academically sound reviews of selected publications. NEPC Reviews are made possible in part by support provided by the Great Lakes Center for Education Research and Practice:

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The mission of the Great Lakes Center for Education Research & Practice is to support and disseminate high-quality research and reviews of research for the purpose of informing education policy and to develop research-based resources for use by those who advocate for education reform.

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Michelle Renée Valladares
(720) 505-1958

Clive Belfield
(718) 997-5440