Risk Priority Number

Risk Priority Number (RPN) is the product of three ratings used in Failure Mode and Effects Analysis (FMEA): Severity (how bad is the effect of the failure?), Occurrence (how often is the failure cause expected to occur?), and Detection (how likely is the failure to be detected before reaching the customer?). Each factor is rated on a 1-10 scale, making RPN range from 1 (lowest risk) to 1,000 (highest risk). Teams set a threshold (commonly 100-200) above which corrective actions are required.

RPN has known limitations. Two failure modes with identical RPNs can have very different risk profiles: Severity=10, Occurrence=2, Detection=5 (RPN=100) represents a catastrophic failure mode with low occurrence, while Severity=2, Occurrence=10, Detection=5 (RPN=100) represents a minor nuisance that occurs frequently. Best practice is to always address any failure mode with Severity≥9 regardless of RPN, because the potential consequences are unacceptable even if occurrence is low.

The 2019 AIAG-VDA FMEA handbook introduced Action Priority (AP) to supplement or replace RPN-based prioritization. AP uses lookup tables that assign High, Medium, or Low priority based on the combination of S, O, and D ratings, ensuring that high-severity failure modes receive appropriate attention regardless of their calculated RPN. Many organizations have transitioned to AP for new FMEAs while maintaining RPN for legacy FMEAs to preserve comparability over time.