Abstract

We consider the problem of off-policy evaluation for reinforcement learning, where the goal is to estimate the expected reward of a target policy \(\pi\) using offline data collected by running a logging policy \(\mu\). Standard importance-sampling based approaches for this problem suffer from a variance that scales exponentially with time horizon \(H\), which motivates a splurge of recent interest in alternatives that break the "Curse of Horizon" (Liu et al. 2018, Xie et al. 2019). In particular, it was shown that a marginalized importance sampling (MIS) approach can be used to achieve an estimation error of order \(O(H^3/ n)\) in mean square error (MSE) under an episodic Markov Decision Process model with finite states and potentially infinite actions. The MSE bound however is still a factor of \(H\) away from a Cramer-Rao lower bound of order \(Ξ©(H^2/n)\). In this paper, we prove that with a simple modification to the MIS estimator, we can asymptotically attain the Cramer-Rao lower

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