Recommender systems are software tools used to generate and provide suggestions for items
and other entities to the users by exploiting various strategies. Hybrid recommender systems
combine two or more recommendation strategies in different ways to benefit from their complementary
advantages. This systematic literature review presents the state of the art in hybrid
recommender systems of the last decade. It is the first quantitative review work completely focused
in hybrid recommenders. We address the most relevant problems considered and present
the associated data mining and recommendation techniques used to overcome them. We also
explore the hybridization classes each hybrid recommender belongs to, the application domains,
the evaluation process and proposed future research directions. Based on our findings, most of
the studies combine collaborative filtering with another technique often in a weighted way. Also
cold-start and data sparsity are the two traditional and top problems being addressed in 23 and
22 studies each, while movies and movie datasets are still widely used by most of the authors.
As most of the studies are evaluated by comparisons with similar methods using accuracy metrics,
providing more credible and user oriented evaluations remains a typical challenge. Besides
this, newer challenges were also identified such as responding to the variation of user context,
evolving user tastes or providing cross-domain recommendations. Being a hot topic, hybrid
recommenders represent a good basis with which to respond accordingly by exploring newer
opportunities such as contextualizing recommendations, involving parallel hybrid algorithms,
processing larger datasets, etc