The Construction and Validation of the OASys Violence Predictor

• Author: Louise Dixon,Philip D. Howard
• Date: 01 March 2012
• Published date: 01 March 2012
• DOI: 10.1177/0093854811431239

287

CRIMINAL JUSTICE AND BEHAVIOR, Vol. 39 No. 3, March 2012 287-307

DOI: 10.1177/0093854811431239

© 2012 International Association for Correctional and Forensic Psychology

AUTHORS’ NOTE: This research was supported by the National Offender Management Service. This article

forms part of Philip Howard’s doctoral thesis. We would like to thank the reviewers for their perceptive

comments on this article. Correspondence concerning this article should be addressed to Philip Howard, 26

Montana Close, South Croydon CR2 0AT, UK; email: philipdavidhoward@gmail.com.

THE CONSTRUCTION AND VALIDATION

OF THE OASYS VIOLENCE PREDICTOR

Advancing Violence Risk Assessment in

the English and Welsh Correctional Services

PHILIP D. HOWARD

LOUISE DIXON

University of Birmingham

The Offender Assessment System (OASys) is the risk assessment and management system routinely used in the National

Offender Management Service (NOMS), the prison and probation service for England and Wales. This study describes the

construction and validation of a new actuarial violence risk measure, the OASys Violence Predictor (OVP), using OASys and

Police National Computer data. Ordinal logistic regression identifies static and dynamic risk factors predictive of violent

recidivism among convicted offenders (N = 15,918). These form the basis of a user-friendly 100-point scale (OVP). OVP

achieves significantly greater predictive validity than existing actuarial scores available within NOMS (the original OASys

risk prediction score; the Offender Group Reconviction Scale, Version 3; and the V scale of the Risk Matrix 2000) on a later

validation sample (N = 49,346). The discussion considers explanations for this improvement, examines the utility of dynamic

risk factors in violence prediction, and describes the application of OVP in the NOMS treatment allocation and risk manage-

ment practice.

Keywords: OASys; violence risk assessment; UK correctional services; dynamic risk factors; AUC

Empirical developments in forensic risk assessment have been pivotal in increasing the

accuracy of decisions about the likely risk an offender poses to the public (Gottfredson

& Moriarty, 2006) and identifying criminogenic factors for intervention (Andrews &

Bonta, 2006). A range of risk assessment tools for violent reoffending is now available

(Hanson, 2005; Heilbrun, Yasuhara, & Shah, 2009). However, research examining the role

of dynamic risk factors (Brown, St. Amand, & Zamble, 2009; Douglas & Skeem, 2005) and

proposals for new measures and approaches (Edens & Douglas, 2006; Howells, 2009;

Walters, 2007) continue to be developed and debated.

Some researchers question whether it is possible to produce new, more accurate predic-

tors of violent recidivism. According to this view, inherent unpredictability in offending

behavior and the criminal justice system impose a “ceiling” on predictive validity that may

now have been reached (Campbell, French, & Gendreau, 2007; Yang, Wong, & Coid,

2010). High levels of intercorrelation and content overlap are found among predictors of

violent reoffending (Campbell et al., 2007; Kroner, Mills, & Reddon, 2005; Yang, Wong,

288 CRIMINAL JUSTICE AND BEHAVIOR

et al., 2010) and actuarial predictors of sexual reoffending. In some circumstances, how-

ever, there is a strong case for the development of a new violence risk assessment instrument.

We describe the construction and validation of such a tool for the National Offender

Management Service (NOMS) of England and Wales. NOMS is responsible for the super-

vision of offenders aged 18 and over serving sentences in custody and the community,

numbering 83,500 (June 2009) and 174,000 (December 2009), respectively (Ministry

of Justice, 2010a). NOMS’s primary risk/needs assessment instrument is the Offender

Assessment System (OASys), a tool that has seldom been studied in forensic psychology

and is described in detail in the Materials section of this article.

OASys has been broadly successful in providing an effective structure for risk assess-

ment with which probation staff is comfortable (Mair, Burke, & Taylor, 2006). NOMS is

committed to staff development and quality assurance procedures that aim to ensure that

offender management practice addresses the risks and needs identified in assessments

(National Probation Service, 2006). However, significant problems relating to predictive

validity have been evident. The original OASys scoring system was shown to be an inad-

equate predictor of general reoffending (Howard, 2006) and, in subsequent unpublished

analysis, violent reoffending. NOMS policy makers mandated that the scoring system

should be revised, with particular attention paid to the prediction of harmful recidivism.

Valid prediction of harmful recidivism is necessary to assess future risks correctly when

reporting to courts (National Probation Service, 2009) and the Parole Board (HM Prison

Service, 2009), determining intensity of probation supervision (National Probation Service,

2008), and allocating places in offending behavior programs (Palmer et al., 2009).

Instruments specifically designed to assess the risk of violent recidivism, including

Historical, Clinical, Risk Management-20 (HCR-20) (Webster, Douglas, Eaves, & Hart,

1997) and Violence Risk Appraisal Guide (VRAG) (Quinsey, Harris, Rice, & Cormier,

1998), are already used to assess some offenders in the NOMS caseload, such as those

serving lengthy prison sentences. However, NOMS policy makers determined that the

additional time required to complete these assessments precluded their general introduction,

given the large caseloads handled by NOMS staff (Ministry of Justice, 2010a) and intensify-

ing budgetary pressures (HM Treasury, 2010). The only practical options for widespread

violence risk prediction in the NOMS caseload would be the use of rapidly scored static

actuarial tools—Offender Group Reconviction Scale, Version 3 (OGRS3; Howard, Francis,

Soothill, & Humphries, 2009) or the V scale of Risk Matrix 2000 (RM2000/V; Thornton, et al.

2003), described below—or the development of a predictor within OASys, as its continued

use by NOMS is certain for the foreseeable future. The decision was therefore made within

NOMS that an OASys-based violence predictor should be developed. Its predictive validity

could then be compared with the original OASys score, OGRS3, and RM2000/V, and a deci-

sion made on whether to implement it as a revision to the operational OASys.

The existing literature offered some evidence for the likely success of an OASys-based

predictor. First, scales from assessment tools designed on risk/needs principles can predict

violence well. For example, Campbell et al. (2007) and Yang, Wong, et al. (2010) found

that Level of Service Inventory-Revised (LSI-R) (Andrews & Bonta, 1995) had similar

validity to several other tools. Campbell et al. also found very large effect sizes for Level

of Service/Case Management Inventory (LS/CMI) (Andrews, Bonta, & Wormith, 2004) in

a small number of studies. Second, this study develops a predictor using a mixture of