Social Recommendation with Missing Not a Random Data

With the explosive growth of online social networks, many social recommendation methods have been proposed and demonstrated that social information has potential to improve the recommendation performance. However, existing social recommendation methods always assume that the data is missing at random (MAR) but this is rarely the case. In fact, by analysing two real-world social recommendation datasets, we observed the following interesting phenomena: (1) users tend to consume and rate the items that they like and the items that have been consumed by their friends. (2) When the items have been consumed by more friends, the average values of the observed ratings will become smaller, not larger as assumed by the existing models. To model these phenomena, we integrate the missing not at random (MNAR) assumption in social recommendation and propose a new social recommendation method SPMF-MNAR, which models the observation process of rating data based on user's preference and social influence. Extensive experiments conducted on large real-world datasets validate that SPMF-MNAR achieves better performance than existing social recommendation methods and the non-social methods based on MNAR assumption.