A1427
Title: Principal stratification with U-statistics under principal ignorability
Authors: Fan Li - Yale University (United States) [presenting]
Abstract: Principal stratification is a popular framework for causal inference in the presence of an intermediate outcome. While the principal average treatment effects are the standard target of inference, they may be insufficient when interest lies in the relative ordering of potential outcomes within a principal stratum. The principal generalized causal effect estimands are introduced to accommodate nonlinear contrast functions, providing robust, probability-scale summaries suitable for ordinal outcomes and win-loss comparisons with composite endpoints. Under principal ignorability, the theoretical results are expanded to a broader class of causal estimands in the presence of a binary intermediate variable. Nonparametric identification results are developed and efficient influence functions are derived for the generalized causal estimands in principal stratification analyses. These efficient influence functions motivate multiply robust estimators and lay the ground for obtaining efficient debiased machine learning estimators via cross-fitting based on U-statistics. The proposed methods are illustrated through simulations and the analysis of a data example.
