In assessing the performance of any plant, works, site or industry, it is essential that we have a measure against which to judge its adequacy. This general term for such a measure is Criterion (plural Criteria). Criteria may be used either in the predictive mode, that is to assess the need for action against some predicted risk, or in the historical mode to assess actual recorded performance. They are generally quantitative statements expressed as the frequency of a specified undesired event. In setting criteria, it is essential that the units chosen are consistent with available techniques for prediction or measurement, and that the chosen value is within the limits of credible use of those techniques, otherwise no valid comparison can be made. In the field of hazard and risk assessment, criteria are set as standards of safety performance, which may be used to indicate situations where expenditure or procedural changes are necessary. An organisation may set criteria for itself or be guided by some external body. Criteria based on risk alone can only provide general guidelines. The accuracy of assessments is generally only to order of magnitude precision and so criteria cannot be applied rigidly. The law in the UK requires adoption of "reasonably practicable" measures to prevent accidents, which means implementing such measures unless the reduction in risk is insignificant in relation to the sacrifice necessary to achieve it. Although this can involve emotive issues, implicitly or explicitly putting a value on human life and suffering, cost-benefit aspects are an important element in taking decisions. However, it is useful to set risk targets for designers in order to encourage development of economical ways of limiting risk. Several ways of including these concepts in risk criteria have been proposed. One approach involves setting a limiting risk criterion, which must be achieved; costs, risk and benefits are considered only once this target has been met. A second, insignificant level of risk may also be set as a very stringent target beyond which further reduction in risk would be unlikely to be justifiable. This type of compound risk criterion is known as a Two Boundary Criterion. The most common criteria used in hazard and risk assessment are those associated with fatal accidents. These can relate to major incidents where multiple fatalities could occur or to those incidents where the possible consequences are limited. In the case of multiple fatalities, these are often expressed as Societal Risk Criteria to give an indication of the impact on the local population of a catastrophic event. Because of the difficulty of accurately predicting the number of fatalities in any particular event, it is sometimes convenient to consider Major Incident Criteria instead. In this case, a number of categories of incident are defined with a range of consequences in each category. The categories are chosen to be consistent with the discriminating power of the assessment technique. Frequency criteria can then be ascribed to each category. Individual Risk Criteria are used where it is necessary to consider the distribution of risk. Individual risk criteria may either be expressed as peak values to indicate where the risk is concentrated on one or a very few individuals or as average values where the risk is shared fairly evenly amongst the exposed population. Both peak and average values have their advantages; Average Individual Risks can be compared directly with statistics for other man-made and natural risk to achieve a good perspective on the size of the problem, while Peak Individual Risks will indicate situations where a small sector of the local population carries a disproportionate amount of the total risk. An example of such a criterion is the Fatal Accident Rate (FAR), which is usually used in assessing risk to an exposed workforce rather than to a population outside a works. FAR is sometimes applied in the predictive mode to risk faced by an individual in a particular job. In this case, it is defined as the predicted number of fatal accidents per 10 hours of exposure to the hazards involved in that job. (10 hours is approximately equal to the working lifetime of a thousand people). Only rarely will any one of these criteria convey the whole picture alone and a thorough assessment will usually examine the situation against a number of them. Terms, which have been widely used in the past, are acceptable risk and criterion of acceptability. At first sight, these seem attractive concepts because they suggest an absolute level of performance which if achieved would be acceptable and therefore would avoid much emotional debate. However the word "acceptable" immediately begs the question "acceptable to whom?" and the debate in this area is no less emotional. The true value of quantitative assessment and criteria is not in trying to prove that a given situation is acceptable, but in improving decision taking by helping to put problems in perspective. Acceptability is a much wider issue involving not just the quantitative assessment and criteria, but also the Perceived Risk as seen by those concerned. Perceived risk is the phenomenon of an individual interpreting the magnitude of a risk against the background of his own understanding. This background may be the result of extrapolation of his own experience or may be influenced by "popular belief" as expressed in the media or by other interested parties. Thus the perceived risk may either exceed or fall short of the result of a quantitative assessment. Perceived risk cannot be predicted and is often linked only loosely to measures taken to reduce the true risk. It will generally be influenced most directly by education or propaganda. Thus acceptability is most unlikely to be determined specifically by hazard and risk assessment and it is therefore strongly recommended that terms in the field of criteria which use the word "acceptable" should be avoided