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Precision of actuarial risk assessment instruments

Evaluating the `margins of error' of group v. individual predictions of violence

Department of Psychology, Simon Fraser University, Burnaby, British Columbia, Canada and Faculty of Psychology, University of Bergen, Norway

Christine Michie, BSc

Department of Psychology, Glasgow Caledonian University

David J. Cooke, PhD, FBPsS, FRSE

Department of Psychology, Glasgow Caledonian University and Forensic Clinical Psychology Services, Greater Glasgow Health Board, Mental Health and Community Trust, Glasgow, Scotland

Correspondence: Professor Stephen D. Hart, Department of Psychology, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6. Email: hart{at}sfu.ca

Declaration of interest None.

Funding detailed in Acknowledgements.

Background Actuarial risk assessment instruments (ARAIs) estimate the probability that individuals will engage in future violence.

Aims To evaluate the `margins of error' at the group and individual level for risk estimates made using ARAIs.

Method An established statistical method was used to construct 95% CI for group and individual risk estimates made using two popular ARAIs.

Results The 95% CI were large for risk estimates at the group level; at the individual level, they were so high as to render risk estimates virtually meaningless.

Conclusions The ARAIs cannot be used to estimate an individual's risk for future violence with any reasonable degree of certainty and should be used with great caution or not at all. In theory, reasonably precise group estimates could be made using ARAIs if developers used very large construction samples and if the tests included few score categories with extreme risk estimates.

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