Grade retention has been controversial for many years, and current calls to end social promotion have lent new urgency to this issue. On the one hand, a policy of retaining in grade those students making slow progress might facilitate instruction by making classrooms more homogeneous academically. On the other hand, grade retention might harm high-risk students by limiting their learning opportunities. Analyzing data from the US Early Childhood Longitudinal Study Kindergarten cohort with the technique of multilevel propensity score stratification, we find no evidence that a policy of grade retention in kindergarten improves average achievement in mathematics or reading. Nor do we find evidence that the policy benefits children who would be promoted under the policy. However, the evidence does suggest that children who are retained learn less than they would have had they instead been promoted. The negative effect of grade retention on those retained has little influence on the overall mean achievement of children attending schools with a retention policy because the fraction of children retained in those schools is quite small. Nevertheless, the effect of retention on the retainees is considerably large.

Defining causal effects as comparisons between marginal population means, this article introduces marginal mean weighting through stratification (MMW-S) to adjust for selection bias in multilevel educational data. The article formally shows the inherent connections among the MMW-S method, propensity score stratification, and inverse-probability-of-treatment weighting (IPTW). Both MMW-S and IPTW are suitable for evaluating multiple concurrent treatments and hence have broader applications than matching, stratification, or covariance adjustment for the propensity score. Furthermore, mathematical consideration and a series of simulations reveal that the MMW-S method has incorporated some important strengths of the propensity score stratification method, which generally enhance the robustness of MMW-S estimates in comparison with IPTW estimates. To illustrate, the author applies the MMW-S method to evaluations of within-class homogeneous grouping in early elementary reading instruction.

This article considers the policy of retaining low-achieving children in kindergarten rather than promoting them to first grade. Under the stable unit treatment value assumption (SUTVA) as articulated by Rubin, each child at risk of retention has two potential outcomes: Y(1) if retained and Y(0) if promoted. But SUTVA is questionable, because a child's potential outcomes will plausibly depend on which school that child attends and also on treatment assignments of other children. We develop a causal model that allows school assignment and peer treatments to affect potential outcomes. We impose an identifying assumption that peer effects can be summarized through a scalar function of the vector of treatment assignments in a school. Using a large, nationally representative sample, we then estimate (1) the effect of being retained in kindergarten rather than being promoted to the first grade in schools having a low retention rate, (2) the retention effect in schools having a high retention rate, and (3) the effect of being promoted in a low-retention school as compared to being promoted in a high-retention school. This third effect is not definable under SUTVA. We use multilevel propensity score stratification to approximate a two-stage experiment. At the first stage, intact schools are blocked on covariates and then, within blocks, randomly assigned to a policy of retaining comparatively more or fewer children in kindergarten. At the second stage, "at-risk" students within schools are blocked on covariates and then assigned at random to be retained. We find evidence that retainees learned less on average than did similar children who were promoted, a result found in both high-retention and low-retention schools. We do not detect a peer treatment effect on low-risk students.