How to manage missing covariates in randomized controlled trials: a comparison of strategies

This article clarifies the assumptions and mechanisms by which different missing data strategies accommodate missingness in covariates and reconcile recommendations that sometimes appear contradictory in the literature. Under MAR, "MI by arm" produces unbiased estimates for both the average treatment effect and subgroup treatment effects. Leveraging the randomization design, "baseline-only MI", grand mean imputation, and the missing indicator method produce unbiased estimates for the average treatment effect, but biased subgroup treatment effects, regardless of the missing data mechanism.