Risk Assessment: Theory, Methods, and Applications

CHAPTER 4

HOW TO MEASURE AND EVALUATE RISK

You cannot solve a problem until you can measure it.

—Daniel Patrick Moynihan

4.1 INTRODUCTION

Risk assessments are usually performed to provide input to a decision. The decision may, for example, concern modification 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 will ultimately determine what information we can get from a risk analysis and the validity of our conclusions.

In this chapter we consider how to quantify and evaluate risk to humans. We first introduce various indicators for expressing quantities of risk, before discussing qualitative and quantitative principles for evaluating whether the risk is acceptable.

Acceptable risk is a compound expression that has been disputed since the late 1960s, when societal debates incited the general public on issues of facility siting and technology development. Fischhoff et al. (1981) concluded that no risk is acceptable in isolation:

Strictly speaking, one does not accept risks. One accepts options that entail some level of risk among their consequences.

4.2 RISK INDICATORS

A quantitative assessment of the risk related to a system or activity has to be based on one or more indicators. An indicator is, in this context, a quantity that provides information about the level of risk (Øien, 2001). There are two main types of indicators: ...