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Actuarial Assessment of Violence Risk

To Weigh or Not to Weigh?

Centre for Violence Prevention, Karolinska Institute, Stockholm, Sweden, Martin.Grann{at}cvp.se

Niklas Långström

Centre for Violence Prevention, Karolinska Institute, Stockholm, Sweden

The assigning of different weights to risk factors in actuarial formulas for the assessment of violence risk in criminal offenders has been debated. The authors explore the predictive validity of an index with 10 well-established risk factors for criminal recidivism with respect to violent reconvictions among 404 former forensic psychiatric examinees in Sweden. Four different weighting conditions are tested experimentally, including Nuffield’s method, bivariate and multivariate logistic regression, and an artificial neural network procedure. Simpler weighting techniques do not improve predictive accuracy over that of a nonweighted reference, and the more complex procedures yield a statistical shrinkage effect. The authors hypothesize that the general lack of causal risk factors in prediction models may contribute to the observed low utility of weighting techniques.

Key Words: violence • risk assessment • prediction • forensic psychiatry • artificial neural network • methodology • VRAG • HCR-20

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