Project STAR — Small Class Size
Tennessee Department of Education · Tennessee, USA · 1989
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
Intervention
Random assignment to small class (13–17), regular class (22–25), or regular + teacher aide
Assignment
Randomized controlled trial (student and teacher)
Sample size
11,600 students, 330 teachers, 79 schools
Primary outcome
Standardized test scores through grade 3; long-term economic outcomes
Effect estimate
+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
Decision
Tennessee reduced statewide class sizes in K-3; similar policies adopted in multiple states
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
Institution
Tennessee Department of Education
Location
Tennessee, USA
Year
1989
Policy area
Education
Mechanism
Human capital