Abstract. Graphical structures such as Bayesian networks or M arkov networks are very useful tools for representing irrelevance or independency relationships, and they may be used to e ciently perform reasoning tasks. Singly connected networks are important speci® c cases where there is no more than one undirected path connecting each pair of variables. The aim of this paper is to investigate the kind of properties that a dependency model must verify in order to be equivalent to a singly connected graph structure, as a way of driving automated discovery and construction of singly connected networks in data. Th e main results are the characterizations of those dependency models which are isomorphic to singly connected graphs (either via the d-separation criterion for directed acyclic graphs or via the separation criterion for undirected graphs), as well as the development of e cient algorithms for learning singly connected graph representations of dependency models.Keywords : graphical models, independency relationships, learning algorithms, singly connected networks. 14 February 1997 ; revision received 11 June 1997 ; accepted 12 June 1997 1. Introduction Graphical models have become common knowled ge representation tools capable of e ciently representing and handling independency relationships as well as uncertainty in our knowledge. They comprise a qualitative and a quantitative component. The qualitative component is a graph displaying dependency} independency relationships : the absence of some links means the existence of certain conditional independency relationships between variables, and the presence of links may represent the existence of direct dependency relationships (if a causal interpretation is given, then the (directed) links signify the existence of direct causal in¯uences between the linked variables). Th is is important because an appropriate use of independency or irrelevance relationships is crucial for the management of information, since independency can modularize knowledge in such a way that we only need to consult the pieces of information relevant to the speci® c question in which we are interested, instead of having to explore a whole knowledge base. Th e quantitative component is a collection of numerical parameters, usually conditional probabilities, which give idea of the strength of the dependencies and measure our uncertainty. Therefore,
Received