Fault Tree Analysis

Origin. Bell Labs (1962) for Minuteman missile system; widely adopted in nuclear, aerospace, and chemical industries.

Mechanism. Works backward from an undesired top event through gates (AND, OR) to basic events. The tree structure makes explicit whether failures must combine (AND) or whether any single failure suffices (OR). Quantitative analysis computes top-event probability from basic-event probabilities.

Procedure. Define the top event (the undesired outcome). Ask what immediate causes could produce it. For each, determine the gate type: AND (all must occur) or OR (any suffices). Recurse until reaching basic events (things that can be directly measured or estimated). Calculate the probability of the top event by propagating probabilities through the tree.

Applies to. Safety analysis, reliability engineering, risk assessment for systems with known structure.

Limitations. The tree encodes the analyst's causal model; causes not imagined are not in the tree. Common-cause failures (one event that triggers multiple branches) are easy to miss. The probabilities assigned to basic events are often guesses dressed as data.