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Fault Tree Analysis Glossary

Plain-English definitions of the terms that come up when you actually do fault-tree analysis. Algorithms (MOCUS), structural concepts (minimal cut sets), importance measures (Birnbaum, Fussell-Vesely, RAW, RRW), hardware metrics (PMHF), and dependency models (Beta-factor CCF). Each entry includes the formula where it matters and a worked example you can sanity-check against.

Algorithms & structure

MOCUS algorithm

Method for Obtaining Cut Sets — the canonical top-down procedure for enumerating minimal cut sets.

Minimal cut sets

The smallest combinations of basic-event failures that, occurring together, cause the top event.

Importance measures

Birnbaum importance

∂P(top)/∂P(event) — the marginal contribution of one basic event to the top-event probability.

Fussell-Vesely importance

Fraction of top-event probability attributable to cut sets containing a given basic event.

Risk Achievement Worth (RAW)

Ratio by which top-event probability rises if a basic event is forced to fail.

Risk Reduction Worth (RRW)

Ratio by which top-event probability falls if a basic event is made perfectly reliable.

Hardware metrics & dependencies

Probabilistic Metric for random Hardware Failures (ISO 26262) — top-event probability per hour, ASIL targets attached.

Beta-factor CCF

Simple parametric model for common-cause failure of redundant components: total λ split into independent and common parts.