  Collaborative filtering (CF) has become a popular method for developing
recommender systems (RSs) where ratings of a user for new items are predicted
based on her past preferences and available preference information of other
users. Despite the popularity of CF-based methods, their performance is often
greatly limited by the sparsity of observed entries. In this study, we explore
the data augmentation and refinement aspects of Maximum Margin Matrix
Factorization (MMMF), a widely accepted CF technique for rating predictions,
which has not been investigated before. We exploit the inherent characteristics
of CF algorithms to assess the confidence level of individual ratings and
propose a semi-supervised approach for rating augmentation based on
self-training. We hypothesize that any CF algorithm's predictions with low
confidence are due to some deficiency in the training data and hence, the
performance of the algorithm can be improved by adopting a systematic data
augmentation strategy. We iteratively use some of the ratings predicted with
high confidence to augment the training data and remove low-confidence entries
through a refinement process. By repeating this process, the system learns to
improve prediction accuracy. Our method is experimentally evaluated on several
state-of-the-art CF algorithms and leads to informative rating augmentation,
improving the performance of the baseline approaches.
