Risk Priority Number (RPN)

The RPN provides a simple, numerical method to rank the potential failure modes identified during an FMEA session. The three components are critical to its calculation. ‘Severity’ (S) assesses the seriousness of the end effect of the failure, with 1 being ‘no effect’ and 10 being ‘catastrophic failure with potential fatality’. ‘Occurrence’ (O) estimates the likelihood that a specific cause will occur and result in the failure mode, where 1 is ‘extremely unlikely’ and 10 is ‘inevitable’. ‘Detection’ (D) evaluates the ability of current controls to detect the cause or the failure mode before it reaches the customer, with 1 being ‘certain detection’ and 10 being ‘cannot be detected’.

While powerful, the RPN has known limitations. A key criticism is that different combinations of S, O, and D can yield the same RPN, yet represent vastly different risk profiles (e.g., S=10, O=2, D=2 gives RPN=40, and S=4, O=5, D=2 also gives RPN=40). The first case is a high-severity issue that should likely be addressed regardless of its low RPN. This has led to the development of alternative prioritization methods, such as the Action Priority (AP) tables in the AIAG-VDA FMEA handbook, which prioritize actions based on combinations of S, O, and D rather than just their mathematical product.