Guest Column | April 7, 2021 Rating Risk Events: Why We Should Replace The Risk Priority Number (RPN) With The Adjusted Risk Likelihood (ARL) 1 ISO 31000 describes a risk as the “effect of uncertainty on objectives.” 2 Both definitions include the significance of managing both a risk event’s uncertainty and impact. Thus, any risk rating system must preserve both attributes to the maximum extent possible. The most common approach for rating risk events in the medical device and pharmaceutical industries is identified in ICH Q9 by failure mode and effect analysis (FMEA). 3 FMEA rates risks using a risk priority number (RPN). Because the RPN has been widely used by the industry for many years, it has shaped the fundamental understanding of how a risk event’s severity and uncertainty are viewed and understood. The RPN is calculated as the mathematical product of numerical ratings from 1 to N of both a risk event’s severity (S) and likelihood of occurrence (L). In some cases, a detection rating (D), also rated from 1 to N, is included. The RPN is basically a severity rating with the numerical likelihood value used as a coefficient to increase the rating value for risks that have a higher likelihood of occurrence. The approach is generally valid because a more likely risk should be assigned a higher severity to increase its priority. The RPN does not have an absolute value because the Ns used for S, L, and D vary from one analysis to another. Thus, any specific RPN value has little meaning, eliminating its general utility to describe the importance of a risk event. In practice, a risk event’s RPN is never seen outside the context of the risk analysis that produced it.