Examining the Predictive Validity of the SORAG in Switzerland

Abstract

Background:The SORAG is one of the most commonly used actuarial risk assessment tools for sex offender evaluations. Although many studies have investigated the predictive validity of this instrument, few have examined whether its published expected rates of recidivism are useful in practice. The aim of the present investigation was to investigate predictive validity of the SORAG in Switzerland, considering both the discrimination and calibration components of predictive validity.Material and Methods:The instrument was administered to two total cohorts of offenders (N = 137) in the Canton of Zurich, Switzerland that were followed for a fixed period of seven years after discharge. Recidivism was defined as new charges and/or convictions for violent (including sexual) offenses. Discrimination was measured using receiver operating characteristic (ROC) curve analysis and calibration using Sanders' decomposition of the Brier score. A dependent t-test was used to examine the difference in expected and observed percentiles, and a Kolmogorov-Smirnov Test was conducted to compare the distribution of offenders in risk bins.Results:ROC analyses revealed an acceptable level of discrimination for both SORAG total risk scores (AUC = 0.69, 95% CI = 0.56-0.82) and risk bins (AUC = 0.67, 95% CI = 0.54-0.80). The seven year recidivism rates both overall as well as for each risk bin were considerably lower than the published SORAG norms. An average forecast error of 20.9% for each risk bin of the SORAG suggests a large difference between expected and observed recidivism rates. Conclusion: Should further studies in Switzerland replicate the current findings, a re-calibration of the SORAG may be needed before the instrument can be considered a valid method to assess recidivism risk in Swiss sex offender populations.