24.5.1.1 Heuristic approaches
There has been some work done to extend the similarity computation of the traditional heuristic-based collaborative filtering technique to reflect multi-criteria rating
information [2, 49, 92]. In this approach, the similarities between users are computed by aggregating traditional similarities from individual criteria or using multidimensional distance metrics.
In particular, the neighborhood-based collaborative filtering recommendation
technique predicts unknown ratings for a given user, based on the known ratings
of the other users with similar preferences or tastes (i.e., neighbors). Therefore, the
first step of the prediction processes is to choose the similarity computation method
to find a set of neighbors for each user. Various methods have been used for similarity computation in single-criterion rating recommender systems, and the most
popular methods are correlation-based and cosine-based. Assuming that R(u, i) represents the rating that user u gives to item i, and I(u, u′) represents the common
items that two users u and u′ rated, two popular similarity measures can be formally
written as follows: