Offline Contextual Bandits with Overparameterized Models
This addresses the problem of algorithm selection and performance in offline reinforcement learning for researchers and practitioners, highlighting a key limitation in policy-based methods.
The paper investigates whether overparameterized models generalize well in offline contextual bandits, finding that value-based algorithms benefit similarly to supervised learning, but policy-based algorithms do not, due to differences in action-stability of their objectives, with formal proofs and experiments showing significant performance gaps.
Recent results in supervised learning suggest that while overparameterized models have the capacity to overfit, they in fact generalize quite well. We ask whether the same phenomenon occurs for offline contextual bandits. Our results are mixed. Value-based algorithms benefit from the same generalization behavior as overparameterized supervised learning, but policy-based algorithms do not. We show that this discrepancy is due to the \emph{action-stability} of their objectives. An objective is action-stable if there exists a prediction (action-value vector or action distribution) which is optimal no matter which action is observed. While value-based objectives are action-stable, policy-based objectives are unstable. We formally prove upper bounds on the regret of overparameterized value-based learning and lower bounds on the regret for policy-based algorithms. In our experiments with large neural networks, this gap between action-stable value-based objectives and unstable policy-based objectives leads to significant performance differences.