… the defining of risk is essentially a political act.

—Roger E. Kasperson


This chapter has the same title as Stan Kaplan's talk to the Annual Meeting of the Society of Risk Analysis in 1996 when he received the Society's Distinguished Award. The talk was later published in the journal Risk Analysis and has become one of the most influential articles on risk concepts (Kaplan, 1997).

The risk concept was introduced briefly in Chapter 1. When we ask “What is the risk?”, we really ask three questions: (i) What can go wrong? (ii) What is the likelihood of that happening? and (iii) What are the consequences?

To be able to answer these questions, we first need to clearly define what we mean by the words used in the questions, and we also need to define several other, associated terms.


To be able to answer the first question, we need to specify what we mean by “What can go wrong?” The answer must be one or more events, or sequences of events. An event is defined as:

image Event: Incident or situation which occurs in a particular place during a particular interval of time (AS/NZS 4360, 1995).

In this book, the term event is used to denote a future occurrence. The duration of the event may range from very short (e.g., an instantaneous shock) to a rather long period.

We distinguish between two types of events that ...

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