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Version 1.2 – Updated June 2023

Reducing class size


  • Influence: Reducing class size
  • Domain: Classroom
  • Sub-Domain: Class composition effects
  • Potential to Accelerate Student Achievement: Likely to have a small positive impact
  • Influence Definition: Reduces the number of students in the class often with the aim of increasing the number of individualized student-teacher interactions to improve student learning.


  • Number of meta-analyses: 9
  • Number of studies: 181
  • Number of students: 562,375
  • Number of effects: 1,242
  • Weighted mean effect size: 0.17
  • Robustness index: 3


Journal Title Author First Author's Country Article Name Year Published Variable Number of Studies Number of Students Number of Effects Effect Size
Educational Evaluation and Policy Analysis Glass & Smith USA Meta-analysis of research on class size and achievement 1979 Class size 77 520,899 725 0.09
Elementary School Journal McGiverin, Gilman, & Tillitski USA A meta-analysis of the relation between class size and achievement. 1989 Class size 10 0 24 0.34
Journal of the Royal Statistical Society. Series C: Applied Statistics Goldstein, Yang, Omar, Turner, & Thompson UK meta-analysis using multilevel models with an application to the study of class size effects 2000 Class size 9 29,440 36 0.20
Campbell Systematic Reviews Dalgaard, Bondebjerg, Klokker, Viiholt, & Dietrichson Denmark Adult/child ratio and group size in early childhood education or care to promote the development of children aged 0–5 years: A systematic review.  2022 Class size in early childhood 5 6,256 84 0.10
Educational Evaluation and Policy Analysis Bowne, Magnuson, Schindler, Duncan, & Yoshikawa USA A Meta-Analysis of Class Sizes and Ratios in Early Childhood Education Programs: Are Thresholds of Quality Associated With Greater Impacts on Cognitive, Achievement, and Socioemotional Outcomes? 2017 Class size in early childhood 38 0 328 0.31
Report Leuven & Oosterbeek Netherlands Class size and student outcomes in Europe 2018 Reducing class size by 10 16 0 16 0.10
Campbell Collaboration Filges, Sonne-Schmidt, & Nielsen Denmark Small Class Sizes for Improving Student Achievement in Primary and Secondary Schools: A Systematic Review 2018 Class size 6 0 9 0.11
Journal of Research in Reading Petscher Canada A meta-analysis of the relationship between student attitudes towards reading and achievement in reading 2010 Class size in early childhood 3 5,780 3 0.06
KEDI Journal of Educational Policy Shin & Chung Korea Class Size and Student Achievement in the United States: A Meta-Analysis 2009 Class size 17 0 17 0.20
TOTAL/AVERAGE 181 562,375 1,242 0.17
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