Estimating Treatment Effects from Contaminated Multi-Period Education Experiments: The Dynamic Impacts of Class Size Reductions

This paper introduces an empirical strategy to estimate dynamic treatment effects in randomized trials that provide treatment in multiple stages and in which various noncompliance problems including attrition and selective transitions between treatment and control groups arise. Our approach is applied to the highly influential four year randomized class size study, Project STAR. We find benefits from small class attendance initially in all cognitive subject areas in kindergarten and the first grade. We do not find any statistically significant dynamic benefits from continuous treatment versus never attending small classes in either the second or third grade. Finally, statistical tests confirm that one should account for both selective attrition and noncompliance with treatment assignment.