Policy Improvement for POMDPs Using Normalized Importance Sampling
This addresses the challenge of policy evaluation in partially observable environments for reinforcement learning practitioners, though it appears incremental as it builds on existing importance sampling techniques.
The paper tackles the problem of estimating expected return in POMDPs without prior knowledge, using a new normalized importance sampling method that allows experience from arbitrary policies. It shows an order of magnitude reduction in trials compared to REINFORCE algorithms when used in greedy search.
We present a new method for estimating the expected return of a POMDP from experience. The method does not assume any knowledge of the POMDP and allows the experience to be gathered from an arbitrary sequence of policies. The return is estimated for any new policy of the POMDP. We motivate the estimator from function-approximation and importance sampling points-of-view and derive its theoretical properties. Although the estimator is biased, it has low variance and the bias is often irrelevant when the estimator is used for pair-wise comparisons. We conclude by extending the estimator to policies with memory and compare its performance in a greedy search algorithm to REINFORCE algorithms showing an order of magnitude reduction in the number of trials required.