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.