In this paper, we argue that causal effect models for realistic individualized treatment rules represent an attractive tool for analyzing sequentially randomized trials. Unlike a number of methods proposed previously, this approach does not rely on the assumption that intermediate outcomes are discrete or that models for the distributions of these intermediate outcomes given the observed past are correctly specified. In addition, it generalizes the methodology for performing pairwise comparisons between individualized treatment rules by allowing the user to posit a marginal structural model for all candidate treatment rules simultaneously. If only a small number of candidate treatment rules are under consideration, a non-parametric marginal structural can be used to conveniently carry out all of the pairwise comparisons of interest in a single step. An appropriately chosen marginal structural model becomes particularly useful, however, as the number of candidate treatment rules increases, in which case an approach based on individual pairwise comparisons would be likely to suffer from too much sampling variability to provide an informative answer. In addition, such causal effect models represent an interesting alternative to methods previously proposed for selecting an optimal individualized treatment rule in that they give the user a sense of how the optimal outcome is estimated to change in the neighborhood of the identified optimum. We discuss an inverse-probability-of-treatment-weighted (IPTW) estimator for these causal effect models that is straightforward to implement using standard statistical software and develop an approach for constructing valid asymptotic confidence intervals based on the influence curve of this estimator. The methodology is illustrated in two simulation studies that are intended to mimic an HIV/AIDS trial.
Bembom, Oliver and van der Laan, Mark J., "Analyzing Sequentially Randomized Trials Based on Causal Effect Models for Realistic Individualized Treatment Rules" (May 2007). U.C. Berkeley Division of Biostatistics Working Paper Series. Working Paper 216.