Terrorist risk evaluation using a posteriori fault trees

Analysis of a terrorist attack involves an event that has already occurred and an a posteriori fault tree can be utilized. This differs from a conventional a priori fault tree used for prediction. Since the top event has a known probability of unity, only ratios of the lower event probabilities are needed to solve and these are easier to estimate. If one assumes that probability ratios are the same for both types of fault trees, then a posteriori analysis aids a priori analysis. Use of a sequence diagram helps direct the work of first responders