An assessment of RPN prioritization in a failure modes effects and criticality analysis

The risk priority number (RPN) methodology for prioritizing failure modes is an integral part of the automobile FMECA technique. The technique consists of ranking the potential failures from 1 to 10 with respect to their severity, probability of occurrence, and likelihood of detection in later tests, and multiplying the numbers together. The result is a numerical ranking, called the RPN, on a scale from 1 to 1000. Potential failure modes having higher RPNs are assumed to have a higher design risk than those having lower numbers. Although it is well documented and easy to apply, the method is seriously flawed from a technical perspective. This makes the interpretation of the analysis results problematic. The problems with the methodology include the use of the ordinal ranking numbers as numeric quantities, the presence of holes making up a large part of the RPN measurement scale, duplicate RPN values with very different characteristics, and varying sensitivity to small changes. Recommendations for an improved methodology are also given.