Robustness and efficiency of Rosenbaum’s rank-based estimator in randomized trials: a design-based perspective

Summary Mean-based estimators of causal effects in randomized experiments may behave poorly if the potential outcomes have a heavy tail or contain outliers. An alternative estimator proposed by Rosenbaum (1993) estimates a constant additive treatment effect by inverting a randomization test using ranks. We develop a design-based asymptotic theory for this rank-based estimator and study its robustness and efficiency properties. We show that Rosenbaum’s estimator is robust against outliers with a breakdown point that uniformly dominates that of any weighted quantile estimator. When pretreatment covariates are available, a regression-adjusted version of Rosenbaum’s estimator uses an agnostic linear regression on the covariates and bases inference on the ranks of residuals. Under mild integrability conditions, we show that this estimator is at most 13.6% less efficient, in the worst case, than the commonly used mean-based regression adjustment method proposed by Lin (2013), and often outperforms it when the residuals have heavy tails. Moreover, under suitable assumptions, Rosenbaum’s regression-adjusted estimator is at least as efficient as the unadjusted one. Finally, we initiate the study of Rosenbaum’s estimator when the constant treatment effect assumption may be violated. To analyse the regression-adjusted estimator, we develop local asymptotics of rank statistics under the design-based framework, which may be of independent interest.