Chapter 6 Measuring Risk
6.1 Introduction
Risk assessments are mainly performed to provide input to decision‐making. The decision may, for example, concern modifications of equipment, allocation of risk reduction expenditures, or siting of a hazardous plant. Common to all is the need to specify what to measure and how to evaluate what has been measured. How we measure risk ultimately determines what information we can get from a risk analysis and the validity of our conclusions.
This chapter deals with how to quantify risk. The main focus is on measuring risk to humans, but some comments are also given on how to measure risk to other assets. Most of the chapter consists of presentation and discussion of various risk measures for expressing quantities of risk.
6.2 Risk Metrics
Recall from Chapter 2 that the term “risk” is used to express our uncertainty about what may happen in the future. Because the future is unknown, we express risk in probabilistic terms by using risk metrics. A risk metric has two parts:
- A clear definition and an explanation of a quantity that provides information about the risk level.
- A measurement procedure and a formula that can be used to determine the numerical value of the quantity when data becomes available.
A risk metric is formally defined as follows:
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