Some of knowledge-based recommender systems [35, 37] can also be classified
into this category, because users can provide their general preferences by building
their own hierarchical taxonomy tree (i.e., where all item features are modeled in
a hierarchical manner) and assigning the relative importance level to each component in the tree. As a result, the systems recommend the most relevant items
according to users’ preferences upon the user-defined multiple attributes of item
taxonomy. Furthermore, some of hybrid recommender systems with knowledgebased approach would also fit in this category, particularly case-based reasoning
recommender systems, where items are represented with multi-criteria content in a
structured way (i.e., using a well-defined set of features and feature values) [88].
These systems allow users to specify their preferences on multi-attribute content of
items in their search for items of interest. For example, several case-based travel
recommender systems [73, 75] filter out unwanted items based on each user’s preferences on multi-attribute content (e.g., locations, services, and activities), and find
personalized travel plans for each user by ranking possible travel plans based on the
user’s preferences and past travel plans of this or similar users. In addition, some
case-based recommender systems [9, 70] allow users to “critique” the recommendation results by refining their requirements as part of the interactive and iterative
recommendation process, which uses various search and filtering techniques to continuously provide the user with the updated set of recommendations. For example,
when searching for a desktop PC, users can critique the current set of provided recommendations by expressing their refined preferences on individual features (e.g.,