More Is Less: When Do Recommenders Underperform for Data-rich Users?

Users of recommender systems tend to differ in their level of interaction with these algorithms, which may affect the quality of recommendations they receive and lead to undesirable performance disparity. In this paper we investigate under what conditions the performance for data-rich and data-poor users diverges for a collection of popular evaluation metrics applied to ten benchmark datasets. We find that Precision is consistently higher for data-rich users across all the datasets; Mean Average Precision is comparable across user groups but its variance is large; Recall yields a counter-intuitive result where the algorithm performs better for data-poor than for data-rich users, which bias is further exacerbated when negative item sampling is employed during evaluation. The final observation suggests that as users interact more with recommender systems, the quality of recommendations they receive degrades (when measured by Recall). Our insights clearly show the importance of an evaluation protocol and its influence on the reported results when studying recommender systems.