• Choice, which concerns the selection of one or more alternatives that can be
considered as more appropriate from all candidate ones;
• Sorting, which refers to the classification of the alternatives into a number of
pre-defined categories;
• Ranking, which involves ranking all the alternatives, from the best one to the
worst;
• Description, which concerns the description of each alternative in terms of how
it performs upon each criterion.
All four types of decision problematics can be considered valid for the recom-
mendation problem:
• Choosing and recommending one or more items as more suitable for a particular
user;
• Classifying (or sorting, as Roy defines it) all available items into pre-defined
categories according to their suitability, e.g., into “recommended for purchase”
and “recommended for viewing” items;
• Ranking all available items from the most suitable to the least suitable ones for
a particular user, and presenting a ranked list of recommendations to the user;
• Describing how suitable a particular item is for a specific user, based on how
it is evaluated upon each criterion. It corresponds to a full analysis of the item
performance upon all criteria, illustrating the suitability of an item for the spe-
cific user (that is, in a personalized manner that aims to help the user to make a
selection).