Ignoring scenarios in risk assessments: Understanding the issue and improving current practice

•The paper clarifies and discusses the issue of ignoring scenarios/events in risk assessments.•It draws attention to unknown knowns and events judged to have negligible probability.•The paper provides guidance on how to best handle the challenge in practise.•A key message is that one should strive for complete event and outcome spaces.•Focus should be placed on the key hypotheses and assumptions made.

In risk assessment we are typically faced with a huge number of potential scenarios and events, and in practise some of these are ignored, either because they are not identified or because of judged low probability. However, a scenario or an event may occur despite being extremely unlikely. Considering a large population of such scenarios and events, the occurrence probability is not necessarily negligible. In this paper we take a closer look at this challenge, the main aim being to clarify the issue and provide some recommendation on how to best handle it in practise. A main conclusion is that the risk assessment should be placed in a sufficiently broad framework, ensuring that the outcome and main event spaces are complete, and sufficient focus is placed on the hypotheses and assumptions supporting the detailed scenarios that are identified.