Root Cause Analysis
The structured investigation of why a problem actually happened — past the immediate symptom, past 'human error', down to the process or system condition that allowed it. The foundation of every credible corrective action.
The standard methods
5 Whys: ask why repeatedly until the answer is a process condition you can change. The machine stopped → the fuse blew → the bearing seized → it wasn't lubricated → the lubrication schedule doesn't cover this machine. Fixing the schedule fixes the future; replacing the fuse alone guarantees a repeat. Fishbone (Ishikawa): brainstorm candidate causes across categories — people, method, machine, material, measurement, environment — then verify which are real. Pareto: when you have many instances, find the vital few causes behind most of the pain.
Method matters less than honesty: the test of a root cause is that removing it prevents recurrence, and that it's something the organization controls.
Why 'human error' is never the answer
People make errors at a roughly constant rate; systems determine whether errors become failures. If one person's slip produced a nonconformity, the systemic questions are: why did the process depend on one person being perfect? why wasn't the error caught downstream? was the workload, interface, or training set up to make the slip likely? Auditors treat “root cause: operator error, action: retrained operator” as the signature of an organization that doesn't investigate.
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