What does “Fault tree analysis” mean?
Short answer
Fault tree analysis (FTA) starts from a bad outcome and maps the combinations of failures that could cause it. OSHA lists it for process hazard analysis.
Also known as: FTA, fault tree
How it works
Start with the top event, such as a wrong-strength dose reaching a patient. Ask what conditions could cause it, and join them with logic gates. An AND gate means all must happen together. An OR gate means any one is enough.
Keep branching until you reach basic events, like a failed device, a missed check or a missing barrier.
The tree then shows single points of failure, where one event alone causes the top event, and the barriers that must all fail. You can add probabilities, but a tree without numbers is still useful.
Example: a surgical fire needs fuel, an oxidizer and an ignition source together, an AND gate. Remove any one and the fire cannot start, so each input becomes a control to review.
Where it is used
OSHA's process safety management standard lists Fault Tree Analysis among the methods an employer may use for a process hazard analysis, next to what-if, checklist, HAZOP and failure mode and effects analysis (29 CFR 1910.119(e)(2)).
The VA's National Center for Patient Safety lists it among its proactive risk assessment methods.
Mix-up: a fault tree does not tell the story of what happened. It models how an outcome could happen, so it helps before an event and after one.
Five whys and the fishbone diagram trace causes of one event. A fault tree handles combinations and barriers.
Frequently asked questions
When should I use a fault tree instead of five whys?
Use a fault tree for high-stakes outcomes where several things must fail together. Five whys fits a simpler event with one chain of cause.
Do I need probabilities to do fault tree analysis?
No. A tree with only logic gates and basic events already shows single points of failure. Probabilities help rank risks when you have good data.
Does OSHA require fault tree analysis?
No single method is required. The process safety management standard says an employer must use one or more listed methods that fit the process. Fault Tree Analysis is one.
Sources
- OSHA 29 CFR 1910.119: Process safety management of highly hazardous chemicals
- VHA National Center for Patient Safety: Proactive risk assessment (HFMEA, fault tree analysis, hierarchy of actions)
Reviewed against the sources above on Oct 5, 2026. Rules change: confirm current requirements with the issuing body or your counsel before relying on any summary.
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