Frequently Asked Question
Process-risk structure
PFMEA examines how a process can create a defect or fail to detect it. Start with a complete process flow including transport, inspection, storage, rework, and setup. For each step, identify the function, failure mode, effect, cause, prevention control, detection control, reaction, and owner. Prevention is generally stronger than relying only on final inspection.
Severity, occurrence, and detection ratings
Scale control: S, O, and D are not universal physical constants. Use the approved rating definitions for the project, customer, industry, and regulatory context. The four anchor points below are an illustrative framework for training and should be expanded into the organization’s controlled 1–10 scale before formal release.
| Rating | Severity (S) | Occurrence (O) | Detection (D) |
|---|---|---|---|
| 1 | No noticeable effect on process output or user | Remote cause under controlled process conditions | Automatic or near-certain detection before escape |
| 4 | Minor defect or inconvenience with no major functional impact | Occasional cause observed in comparable operations | Likely detection by an effective in-process control |
| 7 | Major defect, rework, or loss of a primary process requirement | Frequent cause or unstable process mechanism | Detection is uncertain or occurs late in the route |
| 10 | Safety or regulatory consequence, or critical defect escape | Cause is very frequent or process control is absent | No effective detection before shipment or use |
Severity is assigned from the effect, occurrence from the cause and exposure history, and detection from the effectiveness of current controls. Do not reduce a safety or regulatory severity because occurrence is low. RPN is a prioritization aid and must not replace mandatory controls or engineering judgment.
Rating example: for a drilled hole out of position, a project might assign S = 7 for assembly failure, O = 4 for an occasional fixture-related cause, and D = 6 when detection relies on sampling. The traditional RPN is 7×4×6 = 168. A keyed fixture is a prevention action; in-process gauging is a detection control; the control plan must identify both and define the reaction.
Worked example
If a hole is drilled off-location, the cause may be fixture mislocation, the prevention control may be a keyed fixture, and the detection control may be in-process gauging. The control plan should use the same operation and characteristic identifiers as the PFMEA.