What We Evaluate When We Evaluate Recommender Systems: Understanding Recommender Systems' Performance using Item Response Theory.

Current practices in offline evaluation use rank-based metrics to measure the quality of top-n recommendation lists. This approach has practical benefits as it centres assessment on the output of the recommender system and, therefore, measures performance from the perspective of end-users. However, this methodology neglects how recommender systems more broadly model user preferences, which is not captured by only considering the top-n recommendations. In this article, we use item response theory (IRT), a family of latent variable models used in psychometric assessment, to gain a comprehensive understanding of offline evaluation. We use IRT to jointly estimate the latent abilities of 51 recommendation algorithms and the characteristics of 3 commonly used benchmark data sets. For all data sets, the latent abilities estimated by IRT suggest that higher scores from traditional rank-based metrics do not reflect improvements in modeling user preferences. Furthermore, we show that the top-n recommendations with the most discriminatory power are biased towards lower difficulty items, leaving much room for improvement. Lastly, we highlight the role of popularity in evaluation by investigating how user engagement and item popularity influence recommendation difficulty.