FMEA Risk Priority Number (RPN) is a traditional FMEA scoring method that multiplies severity, occurrence, and detection ratings to help prioritize failure-mode risk for action.
Definition
Risk Priority Number (RPN) is a traditional FMEA risk score calculated by multiplying severity, occurrence, and detection ratings. The common formula is RPN = Severity x Occurrence x Detection. Severity reflects the seriousness of the effect, occurrence reflects likelihood or frequency of the cause, and detection reflects the ability of current controls to detect the failure or cause before impact.
RPN is a prioritization aid, not a perfect risk measure. It helps teams focus discussion, compare failure modes, and track whether actions reduce risk, but it should not override judgment about high-severity failures, customer impact, safety, compliance, or prevention opportunities.
History
RPN became common as FMEA expanded through automotive, aerospace, manufacturing, and quality systems. It gave cross-functional teams a simple numerical way to compare many failure modes and document action priority.
Modern practice increasingly supplements or replaces strict RPN thresholds with Action Priority logic, especially in AIAG/VDA-style FMEA. This change reflects known limitations of RPN, including identical scores from different risk profiles and insufficient emphasis on severity.
When to Use
Use RPN when an organization’s FMEA method calls for severity, occurrence, and detection scoring, when comparing risk before and after actions, or when a simple prioritization metric is needed for a large FMEA. It is most useful as a structured conversation starter.
Do not use RPN as the only decision rule for high-risk situations. A failure mode with severe safety or regulatory effect may require action even if the total RPN is not the highest item on the list.
Step-by-Step
- Define the failure mode. Make sure the failure mode, effect, cause, and controls are specific enough to score.
- Rate severity. Score the seriousness of the effect using the organization’s rating table.
- Rate occurrence. Estimate likelihood based on process history, capability, design margin, field data, or expert evidence.
- Rate detection. Score how well current controls detect the cause or failure before impact.
- Calculate RPN. Multiply severity, occurrence, and detection.
- Review high-severity items separately. Do not let multiplication hide serious effects.
- Prioritize actions. Prefer prevention and occurrence reduction before relying only on detection.
- Update after action. Re-score based on verified changes, not planned actions alone.
- Track residual risk. Confirm that remaining risk is acceptable and linked to the control plan.
Examples
- Process FMEA: A wrong-part assembly has severity 8, occurrence 4, and detection 5, producing RPN 160. A poka-yoke fixture reduces occurrence and detection risk.
- Design FMEA: A seal leak has high severity because it can cause customer failure, so the team acts even though occurrence evidence is limited.
- Supplier risk: An incoming material defect receives a high detection score because current inspection is weak, prompting supplier control improvements.
- Service process: Incorrect eligibility approval is scored for customer impact, expected frequency, and control strength before workflow redesign.
Common Pitfalls
- Using arbitrary thresholds. A fixed RPN cutoff can miss severe failure modes below the threshold.
- Comparing scores across weak rating systems. Inconsistent scoring definitions make RPN unreliable.
- Over-focusing on detection. Better inspection does not remove the cause; prevention is usually stronger.
- Ignoring severity. High-severity items deserve special review regardless of RPN rank.
- Re-scoring planned actions. Risk should be reduced only after actions are implemented and verified.
- Treating RPN as precision. Scores are ordinal estimates, not exact mathematical measurements.