Building large-scale Bayesian networks

Bayesian Networks (BNs) model problems that involve uncertainty. A BN is a directed graph, whose nodes are the uncertain variables and whose edges are the causal or influential links between the variables. Associated with each node is a set of conditional probability functions that model the uncertain relationship between the node and its parents. The benefits of using BNs to model uncertain domains are well known, especially since the recent breakthroughs in algorithms and tools to implement them. However, there have been serious problems for practitioners trying to use BNs to solve realistic problems. This is because, although the tools make it possible to execute large-scale BNs efficiently, there have been no guidelines on building BNs. Specifically, practitioners face two significant barriers. The first barrier is that of specifying the graph structure such that it is a sensible model of the types of reasoning being applied. The second barrier is that of eliciting the conditional probability values, from a domain expert, for a graph containing many combinations of nodes, where each may have a large number of discrete or continuous values. We have tackled both of these practical problems in recent research projects and have produced partial solutions for both that have been applied extensively on a number of real-life applications. In this paper we shall concentrate on this first problem, that of specifying a sensible BN graph structure. Our solution is based on the notion of generally applicable ‘building blocks’, called idioms, which can be combined together into modular sub-nets. These can then in turn be combined into larger BNs, using simple combination rules and by exploiting recent ideas on modular and Object Oriented BNs (OOBNs). This approach, which has been implemented in a BN tool, can be applied in many problem domains. We use examples to illustrate how it has been applied to build large-scale BNs for predicting software safety. We believe our work is a major breakthrough for BN applications. Indeed, we have built working BNs for real applications that we believe are much larger than any previously developed.