Breese et al. [15] implement a Bayesian Network where each node corresponds
to each item. The states correspond to each possible vote value. In the network, each
item will have a set of parent items that are its best predictors. The conditional prob-
ability tables are represented by decision trees. The authors report better results for
this model than for several nearest-neighbors implementations over several datasets.
Hierarchical Bayesian Networks have also been used in several settings as a way
to add domain-knowledge for informationfiltering [78]. One of the issues with hier
archical Bayesian networks, however, is that it is very expensive to learn and update
the model when there are many users in it. Zhang and Koren [79] propose a variation over the standard Expectation-Maximization (EM) model in order to speed up
this process in the scenario of a content-based RS.