Attack trees are related to the established fault tree formalism. Fault tree methodology employs boolean expressions to gate conditions when parent nodes are satisfied by leaf nodes. By including a priori probabilities with each node, it is possible to perform calculate probabilities with higher nodes using Bayes Rule. However, in reality accurate probability estimates are either unavailable or too expensive to gather. With respect to computer security with active participants (i.e., attackers), the probability distribution of events are probably not independent nor uniformly distributed, hence, naive Bayesian analysis is unsuitable.