24.5.2 Engaging Multi-Criteria Ratings during Recommendation
As mentioned above, multi-criteria recommender systems may choose to model a
user’s utility for a given item by including both the overall rating and ratings of
individual item components/criteria or they may choose to include only ratings of
individual criteria. If overall ratings are included as part of the model, the recommendation process in such cases is typically very straightforward: after predicting
all unknown ratings, the recommender system uses the overall rating of items to select the most highly predicted items (i.e., the most relevant items) for each user. In
other words, the recommendation process is essentially the same as in traditional,
single-criterion recommender systems.
However, without an overall rating the recommendation process becomes more
complex, because it is less apparent how to establish the total order of the items. For
example, suppose that we have a two-criterion movie recommender system, where
users judge movies based on their story (i.e., plot) and visual effects. Further, suppose that one movie needs to be chosen for recommendation among the following
two alternatives: (i) movie X, predicted as 8 in story and 2 in visuals, and (ii) movie
Y, predicted as 5 in story and 5 in visuals. Since there is no overall criterion to
rank the movies, it is not easy to judge which movie is better, unless some other
modeling approach is adopted, using some non-numerical (e.g., rule-based) way for
expressing preferences. Several approaches have been proposed in the recommender
systems literature to deal with this problem: some try to design a total order on items
and obtain a single global optimal solution for each user, whereas others take one of
the possible partial orders of the items and find multiple (Pareto optimal) solutions.