Fault Trees vs. Event Trees in Reliability Analysis

Reliability analysis is the study of both the probability and the process of failure of a system. For that purpose, several tools are available, for example, fault trees, event trees, or the GO technique. These tools are often complementary and address different aspects of the questions. Experience shows that there is sometimes confusion between two of these methods: fault trees and event trees. Sometimes identified as equivalent, they, in fact, serve different purposes. Fault trees lay out relationships among events. Event trees lay out sequences of events linked by conditional probabilities. At least in theory, event trees can handle better notions of continuity (logical, temporal, and physical), whereas fault trees are most powerful in identifying and simplifying failure scenarios. Different characteristics of the system in question (e.g., a dam or a nuclear reactor) may guide the choice between fault trees, event trees, or a combination of the two. Some elements of this choice are examined, and observations are made about the relative capabilities of the two methods.