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.