Evidence-based policy is the practice of basing policy decisions on rigorous research evidence, such as randomized experiments. But it is unclear how often evidence-based decisions produce more effective policy. We evaluate an evidence-based policy implemented in 1989-93, after the state of Tennessee completed the famous Project STAR randomized experiment, which showed that reducing average class sizes from 23 to 15 could raise test scores by nearly 0.2 standard deviations (SD). After Project STAR, the state launched Project Challenge, which tried to achieve similar score gains by earmarking $5 million to reduce class sizes in the state’s 17 poorest districts.
We evaluate the effects of Project STAR by applying regression discontinuity and difference in differences analysis to data from district report cards. Our analysis offers no evidence that Project Challenge districts raised test scores, and even raises questions about whether districts reduced class sizes. After Project Challenge, Tennessee’s Basic Education Plan did reduce class sizes, but only by a token amount, from 26 to 25. In this example, it seems that a successful randomized experiment did not lead to successful policy.