The Effects of Class Size on Student Achievement: New Evidence from Population Variation

TLDR

The study examines how class size affects student achievement by leveraging longitudinal population variation across 649 elementary schools. Class size variation is identified through idiosyncratic population changes and discrete enrollment‑triggered jumps in maximum/minimum class size rules. Results show no statistically significant effect of class size on achievement, ruling out even modest impacts of 2–4% of a standard deviation for a 10% reduction.

Abstract

Abstract I identify the effects of class size on student achievement using longitudinal variation in the population associated with each grade in 649 elementary schools. I use variation in class size driven by idiosyncratic variation in the population. I also use discrete jumps in class size that occur when a small change in enrollment triggers a maximum or minimum class size rule. The estimates indicate that class size does not have a statistically significant effect on student achievement. I rule out even modest effects (2 to 4 percent of a standard deviation in scores for a 10 percent reduction in class size).