Between 1980 and 2012, the number of states with publicly funded prekindergarten programs in the United States increased fourfold from 10 to 40 (Barnett, Carolan, Fitzgerald, & Squires, 2012).

A primary rationale for prekindergarten programs is their ability to increase school readiness, that is, to prepare children, especially disadvantaged children, for constructive participation in the learning opportunities provided in kindergarten and beyond (Pianta, Cox, & Snow, 2007). Evidence about the effects of prekindergarten on these school readiness constructs is therefore central to the justification of policies that support public funding for prekindergarten.

Much of the currently available evidence on the causal effects of public prekindergarten programs on school readiness outcomes comes from studies that use a regression-discontinuity design (RDD) with the age cutoff to enter a program in a given year as the basis for assignment to treatment and control conditions.

Because the RDD has high internal validity when its key assumptions are met, these studies appear to provide strong evidence for the effectiveness of these programs. However, there are overlooked methodological problems in the way this design has typically been applied that have the potential to produce biased effect estimates.

We describe these problems, argue that they deserve more attention from researchers using this design than they have received, and offer suggestions for improving future studies.