Metric Definitions

RMSE:      Root Mean Squared Error. Lower values mean better accuracy.
MAE:       Mean Absolute Error. Lower values mean better accuracy.
HR:        Hit Rate; how often we are able to recommend a left-out rating. Higher is better.
cHR:       Cumulative Hit Rate; hit rate, confined to ratings above a certain threshold. Higher is better.
ARHR:      Average Reciprocal Hit Rank - Hit rate that takes the ranking into account. Higher is better.
Coverage:  Ratio of users for whom recommendations above a certain threshold exist. Higher is better.
Diversity: 1-S, where S is the average similarity score between every possible pair of recommendations
           for a given user. Higher means more diverse.
Novelty:   Average popularity rank of recommended items. Higher means more novel.