Project STAR — Small Class Size

Summary

Project STAR is one of the largest and most-cited education RCTs ever conducted. Students randomly placed in small classes outperformed peers in regular-sized classes on standardized tests throughout early grades, with larger gains for disadvantaged students. Long-term follow-up by Raj Chetty and colleagues found that students assigned to better early-grade classrooms had higher earnings and college enrollment rates as adults. The experiment established small class size as one of the few education interventions with rigorous causal evidence.

Research question

"What is the effect of small class sizes (13–17 students) on early-grade student achievement?"

Methodology

Result

Positive

+4 percentile points in small vs. regular class; effect doubled for minority and low-income students; long-term earnings gains documented in follow-up studies

Evidence strength

Strong

Randomized controlled trial with large sample.

Replication status

Partially replicated