Randomization Inference with Sample Attrition

Abstract: Although appealing, randomization inference for treatment effects can suffer from severe size distortion due to sample attrition. We propose new, computationally efficient methods for randomization inference that remain valid under a range of potentially informative missingness mechanisms. We begin by constructing valid p-values for testing sharp null hypotheses, using the worst-case p-value from the Fisher randomization test over all possible imputations of missing outcomes. Leveraging distribution-free test statistics, this worst-case p-value admits a closed-form solution, connecting naturally to bounds in the partial identification literature. Our test statistics incorporate both potential outcomes and missingness indicators, allowing us to exploit structural assumptions-such as monotone missingness-for increased power. We further extend our framework to test non-sharp null hypotheses concerning quantiles of individual treatment effects. The methods are illustrated through simulations and an empirical application.