that meet users’ requirements [9]. The bottleneck of this knowledge-based approach
is that it needs to acquire a knowledge base beforehand, but the obtained knowledge
base helps to avoid cold start or data sparsity problems that pure content-based or
collaborative filtering systems encounter by relying on solely the ratings obtained
by users. Hybrid approaches combine content-based, collaborative filtering, and
knowledge-based techniques in many different ways [10]. Upon more in-depth analysis of the representative MCDM recommender systems surveyed in the previous
section, we discover that the multi-criteria nature of the majority of these systems
can be classified in the following three general categories: