Bayesian Networks from the Point of View of Chain Graphs

The paper gives a few arguments in favour of use of chain graphs for description of proba­ bilistic conditional independence structures. Every Bayesian network model can be equiva­ lently introduced by means of a factorization formula with respect to chain graph which is Markov equivalent to the Bayesian network. A graphical characterization of such graphs is given. The class of equivalent graphs can be represented by a distinguished graph which is called the largest chain graph. The factoriza­ tion formula with respect to the largest chain graph is a basis of a proposal how to represent the corresponding (discrete) probability dis­ tribution in a computer (i.e. 'parametrize' it). This way does not depend on the choice of a particular Bayesian network from the class of equivalent networks and seems to be the most efficient way from the point of view of memory demands. A separation criterion for reading indepen­ dences from a chain graph is formulated in a simpler way. It resembles the well-known d-separation criterion for Bayesian networks and can be implemented 'locally'.