Class size reduction mandates are frequent and invariably justified by studies reporting positive effects on student achievement. Yet other studies report no effects, and the literature as a whole awaits correction for potential publication bias. Moreover, if identification drives results systematically, the relevance of individual studies will vary. We build a sample of 1,767 estimates collected from 62 studies and, for each estimate, codify 43 factors reflecting estimation context. We employ recently developed nonlinear techniques for publication bias correction and Bayesian model averaging techniques that address model uncertainty. The results suggest publication bias among studies featured in top five economics journals, but not elsewhere. The implied class size effect is zero for all identification approaches except Tennessee's Student/Teacher Achievement Ratio project. The effect remains zero for disadvantaged students and across subjects, school types, and countries.

Fig: No publication bias, most precise estimates around zero

Bayesian model averaging

Reference: Opatrny Matej, Havranek Tomas, Irsova Zuzana, Milan Scasny (2023), "Publication Bias and Model Uncertainty in Measuring the Effect of Class Size on Achievement." Charles University, Prague.