Model Selection for Offline Reinforcement Learning: Practical Considerations for Healthcare Settings

Reinforcement learning (RL) can be used to learn treatment policies and aid decision making in healthcare. However, given the need for generalization over complex state/action spaces, the incorporation of function approximators (e.g., deep neural networks) requires model selection to reduce overfitting and improve policy performance at deployment. Yet a standard validation pipeline for model selection requires running a learned policy in the actual environment, which is often infeasible in a healthcare setting. In this work, we investigate a model selection pipeline for offline RL that relies on off-policy evaluation (OPE) as a proxy for validation performance. We present an in-depth analysis of popular OPE methods, highlighting the additional hyperparameters and computational requirements (fitting/inference of auxiliary models) when used to rank a set of candidate policies. To compare the utility of different OPE methods as part of the model selection pipeline, we experiment with a clinical decision-making task of sepsis treatment. Among all the OPE methods, FQE is the most robust to different sampling conditions (with various sizes and data-generating behaviors) and consistently leads to the best validation ranking, but this comes with a high computational cost. To balance this trade-off between accuracy of ranking and computational efficiency, we propose a simple two-stage approach to accelerate model selection by avoiding potentially unnecessary computation. Our work represents an important first step towards enabling fairer comparisons in offline RL; it serves as a practical guide for offline RL model selection and can help RL practitioners in healthcare learn better policies on real-world datasets.