Semi-Supervised Off Policy Reinforcement Learning
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Reinforcement learning (RL) has shown great success in estimating sequential treatment strategies which account for patient heterogeneity. However, health-outcome information is often not well coded but rather embedded in clinical notes. Extracting precise outcome information is a resource intensive task. This translates into only small well-annotated cohorts available. We propose a semi-supervised learning (SSL) approach that can efficiently leverage a small sized labeled data ℒ with true outcome observed, and a large sized unlabeled data 𝒰 with outcome surrogates W. In particular we propose a theoretically justified SSL approach to Q-learning and develop a robust and efficient SSL approach to estimating the value function of the derived optimal STR, defined as the expected counterfactual outcome under the optimal STR. Generalizing SSL to learning STR brings interesting challenges. First, the feature distribution for predicting Y_t is unknown in the Q-learning procedure, as it includes unknown Y_t-1 due to the sequential nature. Our methods for estimating optimal STR and its associated value function, carefully adapts to this sequentially missing data structure. Second, we modify the SSL framework to handle the use of surrogate variables W which are predictive of the outcome through the joint law ℙ_Y, O, W, but are not part of the conditional distribution of interest ℙ_Y| O. We provide theoretical results to understand when and to what degree efficiency can be gained from W and O. Our approach is robust to misspecification of the imputation models. Further, we provide a doubly robust value function estimator for the derived STR. If either the Q functions or the propensity score functions are correctly specified, our value function estimators are consistent for the true value function.
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