General model for the risk priority number in failure mode and effects analysis

- A general model is proposed to represent the relationship among the three factors in risk priority number evaluation.
- The three factors, occurrence, detection, and severity, are considered dependent.
- The unique role of each factor is highlighted.

Failure mode and effects analysis (FMEA) is a structured method used during a given stage of the system life cycle to understand all probable failure modes and the effects of their occurrences. The risk priority number (RPN) is calculated in FMEA to select more critical failure modes by multiplying three factors: occurrence, detection, and severity. In the literature, these three factors are defined qualitatively without any underlying model, and multiple definitions and conflicting interpretations exist for each factor. As the interrelationships between the three RPN factors are not known, previous research has treated each factor as a criterion in multiple criteria decision making, under the assumption that the three factors are independent of each other. In this paper, we present a general model to explain the functional relationship among the three factors. Using the model, we discuss the unique role of each factor for comparing the risk of different failure modes.