Comparing two predictors of sexual recidivism: the Risk Matrix 2000 and the OASys Sexual Reoffending Predictor

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Risk assessment for sex offenders

Earlier this week (26 January 2020), HMPPS published a new report in the MoJ Analytical series: Comparing two predictors of sexual recidivism: the Risk Matrix 2000 and the OASys Sexual Reoffending Predictor. The study, authored by Philip Howard & Helen Wakeling, aimed to inform decisions about the risk assessment of men convicted of sexual offences. It did this by testing the predictive validity for sexual reoffending – the ability to separate lower- and higher-risk individuals – of two actuarial risk instruments cited in the report’s title.

The study

Previous research has found OSP to be a slightly better predictor of proven contact sexual reoffending than a simulated version of RM2000/s. This study re-examined this issue using full RM2000/s scores and minor revisions to OSP’s scoring. It also compared a further OSP scale, developed for indecent image reoffending, with RM2000/s. Finally, the study examined whether length of time without offending in the community (on release from prison) affected contact sexual reoffending rates and whether this should be considered in determining individuals’ risk ratings. It examines these issues using a sample of men released from custody in the 2000s.

A prison sample of 2,728 men who had been convicted of at least one sexual offence and who had been risk assessed for participation in an offending behaviour programme, was utilised, and rates of proven reoffending were examined.

RM2000/s, OSP/Contact (OPS/C) and OSP/Indecent images (OSP/I) risk categories were produced for each individual in the sample, and the predictive validity of the tools were examined. The authors set out the limitations of the study which study include the fact that the sample is historic, and that most had contact sexual offences as their index offence.

Predicting sexual reoffending

Actuarial risk assessment tools – which arrive at an overall risk level using empirically-supported risk factors and mathematical rules, rather than professional judgement – have been shown to be good predictors of proven reoffending. The actuarial tool Risk Matrix 2000/s (RM2000/s) was developed using a large sample of adult males convicted of sexual offences in the United Kingdom (UK) and cross-validated with further UK samples. HMPPS and the Police Service currently use it across England and Wales to assess the sexual recidivism risk of such men.
Although RM2000/s is well evidenced and validated, a new actuarial tool called the OASys Sexual Reoffending Predictor (OSP) was developed by Howard and Barnett (2015) in order to address some concerns with RM2000/s. In its initial form (now OSP/C), OSP focused on those offences which tend to cause the most serious harm to the victims (i.e., contact sexual offences), rather than all sexual recidivism. It now also has an additional scale, OSP/I, which predicts the risk of indecent images of children recidivism.

In addition to its focus on specific offences, OSP was designed to be less resource intensive than RM2000/s, and to be used with all men who have committed sexual offending (RM2000/s cannot be used on those whose sexual offending was confined to childhood, i.e., all offending took place before the age of 16).

The table below, reproduced from the study, examines the distributions and contact sexual and indecent image reoffending failure rates by risk category, at the five-year point using life table methods.

Findings

The study found that the OSP tool was more accurate, particularly for offences relating to indecent images:

  • OSP/C was a slightly better predictor of proven contact sexual reoffending than RM2000/s.
  • By far the best predictor of proven indecent images reoffending was OSP/I.
  • Analysis, based on a small sample, indicated that those contact sexual offenders who had been in the community offence-free for a five-year period had a lower risk of reoffending beyond the five-year point. These results, therefore, lend some support to the recent guidance proposing the reduction of risk by one category for every five years an individual convicted of a sexual offence has been in the community offence-free.

Conclusions

The authors conclude that the results provide further support for the use of OSP over RM2000/s in assessing the risk of proven contact sexual reoffending in men with a sexual offending history. OSP has a number of advantages over RM2000/s: it focuses on specific types of sexual reoffending, it has a simpler scoring process, there is no age restriction on its use and it has better predictive validity for its two outcomes.

 

Thanks to Jen Theodore for permission to use the header image previously published on Unsplash.

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