Pessimistic Decision-Making for Recommender Systems.

Modern recommender systems are often modelled under the sequential decision-making paradigm, where the system decides which recommendations to show in order to maximise some notion of either imminent or long-term reward. Such methods often require an explicit model of the reward a certain context-action pair will yield – for example, the probability of a click on a recommendation. This common machine learning task is highly non-trivial, as the data-generating process for contexts and actions can be skewed by the recommender system itself. Indeed, when the deployed recommendation policy at data collection time does not pick its actions uniformly-at-random, this leads to a selection bias that can impede effective reward modelling. This in turn makes off-policy learning – the typical setup in industry – particularly challenging. Existing approaches for value-based learning break down in such environments. In this work, we propose and validate a general pessimistic reward modelling approach for off-policy learning in recommendation. Bayesian uncertainty estimates allow us to express scepticism about our own reward model, which can in turn be used to generate a conservative decision rule. We show how it alleviates a well-known decision making phenomenon known as the Optimiser’s Curse, and draw parallels with existing work on pessimistic policy learning. Leveraging the available closed-form expressions for both the posterior mean and variance when a ridge regressor models the reward, we show how to apply pessimism effectively and efficiently to an off-policy recommendation use-case. Empirical observations in a wide range of simulated environments show that pessimistic decision-making leads to a significant and robust increase in recommendation performance. The merits of our approach are most outspoken in realistic settings with limited logging randomisation, limited training samples, and larger action spaces. We discuss the impact of our contributions in the context of related applications like computational advertising, and present a scope for future research based on hybrid off-/on-policy bandit learning methods for recommendation.