Paper:
Off-policy training of contextual-bandit policies is attractive in online systems (e.g. search, recommendation, ad placement), since it enables the reuse of large amounts of log data. State-of-the-art methods for off-policy learning, however, are based on inverse propensity score (IPS) weighting, which requires that the logging policy chooses all actions with non-zero probability for any context (i.e., full support). In real-world systems, this condition is often violated, and we show that existing off-policy learning methods based on IPS weighting can fail catastrophically. We therefore develop new off-policy contextual-bandit methods that can controllably and robustly learn even when the logging policy has deficient support. To this effect, we explore three approaches that provide various guarantees for safe learning despite the inherent limitations of support deficient data: restricting the action space, reward extrapolation, and restricting the policy space. We analyze the statistical and computational properties of these three approaches, and empirically evaluate their effectiveness in a series of experiments. We find that controlling the policy space is both computationally efficient and that it robustly leads to accurate policies.

Review:
This paper talks about the problem of off-policy or batch learning in the contextual bandit setting without the complete support assumption. This problem setting is very realistic and encountered in most problems, especially in temporally extended settings, such as reinforcement learning. They compare three approaches for the same: restricting action selection, learning extrapolated reward models, and by restricting the policy class. They derive a SNIPS style estimator for the support constraint in the final approach. The approach with restricting the policy class demonstrates decent empirical results although the direct method is very much comparable.
I would lean towards being mostly neutral in terms of acceptance. While the problem being solved is very relevant and their approach compares three different approaches to the deficient support problem, I am not sure how this work is positioned with respect to approaches solving similar problems in the reinforcement learning land. For example, Batch constrained Q-learning ([1]) restricts the set of actions that can be used, Bootstrapping Error Accumulation ([2]) and SPIBB ([3]) restrict the policy class in batch reinforcement learning. I would appreciate some comparison/positioning to such methods in the bandit setting as well. The support estimation metric and the corresponding objective (Eqn 10, 11) should also be compared and contrast with explicit divergences designed for support matching (for example, in [4]).
References:
[1] Off-Policy Deep reinforcement learning without exploration, Fujimoto et.al. ICML 2019
[2] Stabilizing Off-policy Q-learning via Bootstrapping Error Reduction, Kumar et.al. NeuRIPS 2019
[3] Safe Policy Improvement with Baseline Bootstrapping. Laroche et.al. ICML 2019
[4] Domain adaptation with asymmetrically-relaxed distribution alignment. Wu et.al. ICML 2019