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Violence in its many and varied forms represents a major public health problem. Home Office figures for England & Wales, based on the British Crime Survey, suggest that there were just over two million incidents of violence between January and December 2010.1 It is therefore not surprising that significant efforts have been made to predict the risk of future violence in at risk groups. At a fundamental level, the challenges of assessing risk in this area are similar to those in other areas of public health, where uncertainty can be divided into two types: aleatory and epistemological. Aleatory uncertainty refers to what might also be termed true chance or randomness, as in the throw of a dice, the toss of a coin or the similar types of events which fill statistics textbooks. Epistemological uncertainty refers to our lack of knowledge about potentially verifiable events. In areas such as screening for a disease, it has been noted that the uncertainty is all epistemological, with the disease either being present or absent.2 In making predictions about violence though, there is a mix of the epistemological and aleatory uncertainty. Future events are unknown and chance events will have unknown impacts on these. Gathering more information may change our predictions, but we will always be faced with a remaining level of uncertainty which cannot be eliminated in predicting violence. In common with other areas of prediction, future violence therefore presents very significant challenges.
In addressing these challenges there has, in recent years, been a dramatic growth in the use of structured assessments to try to predict future risk. Most of these assessments have been based on actuarial models of risk and have come to be termed Actuarial Risk Assessment Instruments (ARAIs) or, less formally, risk assessment ‘tools’. These ARAIs have sought to …
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