Large randomized controlled trial finds state pre-k program has adverse effects on academic achievement

straight talk on evidence:

In this report we discuss newly-published findings from a randomized controlled trial (RCT) of Tennessee’s voluntary prekindergarten (pre-k) program for low-income children (Lipsey, Farran, and Durkin 2018). We are highlighting this study for two reasons. First, the effectiveness of state and local pre-k programs is a topic of high policy importance. Approximately 28 percent of the nation’s four-year-olds are enrolled in pre-k programs funded by states, municipalities, or school districts—a number that has grown rapidly over time (Chaudry and Datta 2017)—and policy officials often tout pre-k as a powerful tool for closing school achievement gaps between minorities and whites and increasing earnings later in life (e.g., Executive Office of the President 2015).

Second, this study provides uniquely credible evidence on the topic. It is the first large RCT of a state-funded pre-k program, and one of only two such studies ever conducted of public preschool programs—the other being the national RCT of the federal Head Start program. Other studies of public or private preschool programs have had weaknesses that limit the reliability of their findings, such as lack of random assignment (e.g., Oklahoma universal pre-k, Chicago Child-Parent Centers) or small samples and imperfect randomization (e.g., Perry Preschool Project, Abecedarian Project).

What did the Tennessee study find? Like the Head Start RCT, it found positive effects on student achievement at the end of the pre-k year (e.g., their ability to identify letters and words), but these effects dissipated as children entered elementary school and—in the case of Tennessee—turned modestly negative by third grade, with the control group outperforming the pre-k group in math and science achievement. Here’s a brief overview of the newly-published third-grade findings in Tennessee: