Prediction of violent reoffending in people released from prison in England: external validation study of a risk assessment tool (OxRec)

Purpose: We aimed to externally validate the Oxford Risk of Recidivism (OxRec) tool to estimate 1- and 2-year risk of violent reoffending in people released from prison in England. Methods: We identified people released from prison using administrative data shared between official prison and police services. We extracted information on criminal history, clinical and sociodemographic risk predictors, and outcomes. The outcomes were violent reoffending at 1 and 2 years after release from prison, identified using official police data. Predictive ability was examined using measures of calibration (calibration statistics and plots) and discrimination (area under the receiver operating characteristic curve [AUC]; sensitivity, specificity, positive and negative predictive values [PPV; NPV]) for predetermined risk thresholds. Recalibration of the model was conducted when necessary. Results: In total, 1,770 individuals (median age = 33 [IQR 27–40]; 92% were male) were identified as recently released from prison. 31% (n = 550) and 43% (n = 765) reoffended within 1 and 2 years, respectively. Simple validation of the original model found a systematic underestimation of the probability of reoffending. However, after recalibration, OxRec was associated with AUCs of 0.71 (95% CI: 0.69-0.74) for 1-year and 0.71 (0.68-0.74) for 2-year follow-up. At a pre-specified threshold of 40% for 2-year violent reoffending risk, sensitivity was 77% (95% CI: 74%-80%), specificity 54% (51%-58%), PPV 56% (53%-59%) and NPV 76% (73%-79%). In addition, in the revised model, measures of calibration were good (calibration in the large was null for both time points). Conclusions: External validations of risk assessment tools for reoffending with adequate sample sizes can be conducted using linked data between prison and police services, and may require model recalibration before implementation. In this validation, OxRec had good performance on measures of discrimination and calibration. It can be considered as part of approaches to improve decision-making about risk of serious offending and the allocation of resources in the criminal justice system.