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 information filtering [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 varia- tion over the standard Expectation-Maximization (EM) model in order to speed up this process in the scenario of a content-based RS.