The likelihood of a specified undesired event occurring within a specified period or in specified circumstances. It may be either a frequency (the number of specified event occurring in unit time) or a probability (the probability of a specified event following a prior event), depending on the circumstances. When considering the risk of harm to populations exposed to hazards, it is helpful to consider two derivatives to risk. In cases where the potential is large, many factors dictate the severity, which might be realised, and there is a wide spectrum of possible harmful outcomes with associated likelihood's. This is known as the Societal Risk. In this case, the undesired event in our definition of risk is an accident, which can affect a group of people. This is usually quantified as an F-N curve. Individuals amongst the population who could be affected by such an accident will not usually be exposed equally. This distribution of the risk is illustrated by considering the likelihood of particular individuals being affected, known as the Individual Risk. In this case, the undesired event in our definition of risk is harm to a specific individual (or person living at a particular location). Individual risk can, of course, be used in the limiting case where only one individual could be affected in an accident