Glossary term
Risk Priority Number
A ranking value used in FMEA by combining severity, occurrence, and detection scores.
Definition
metricRisk Priority Number is a prioritisation score used in FMEA, typically calculated from severity, occurrence, and detection rankings.
Risk Priority Number, commonly abbreviated RPN, is used in failure mode and effects analysis to help rank failure modes for review and mitigation. The classic form multiplies severity, occurrence, and detection scores, but the result is an ordinal prioritisation aid rather than a physically meaningful measure of absolute risk.
In a traditional FMEA, Risk Priority Number is calculated as:
where S is severity, O is occurrence, and D is detection. Each score is usually assigned on a ranked scale such as 1 to 10. High severity means serious consequence if the failure occurs. High occurrence means the failure is expected more often. High detection score usually means the failure is less likely to be detected before causing harm, although exact scoring conventions must be defined by the organisation.
Engineering use
RPN helps teams decide which failure modes deserve mitigation, test coverage, design change, supplier control, diagnostic monitoring, or management attention. It is useful as a workshop tool because it forces cross-functional discussion about effect, cause, control, and verification.
The number should not be used mechanically. Severity often deserves special treatment: a low-occurrence but catastrophic hazard may require action even if its RPN is lower than a frequent nuisance failure. Many organisations therefore combine RPN with severity thresholds, action priority tables, risk matrices, or regulatory requirements.
Limitations
RPN multiplies ordinal scales, so equal numerical differences do not necessarily represent equal differences in risk. Different combinations can produce the same RPN while implying very different engineering responses. For example, high severity with low occurrence is not equivalent to low severity with high occurrence, even if the product matches.
The score is also sensitive to subjective interpretation. Two teams can assign different values unless scoring criteria, evidence, assumptions, and review rules are explicit. RPN is best treated as a structured prioritisation signal, not as a calculation that proves a design is safe.
Common mistakes
A common mistake is reducing only the detection score and claiming the risk has been eliminated, while the failure cause and severity remain unchanged. Another is using RPN to hide high-severity hazards below an arbitrary threshold. A strong FMEA review states scoring rules, evidence behind each rating, existing controls, recommended actions, responsible owner, due date, residual score, and whether independent validation is required.