August 2, 2012

Violence risk instruments overpredicting danger

Tools better at screening for low risk than pinpointing high risk 

The team of Seena Fazel and Jay Singh are at it again, bringing us yet another gigantic review of studies on the accuracy of the most widely used instruments for assessing risk of violence and sexual recidivism.

This time, the prolific researchers -- joined by UK statistician Helen Doll and Swedish professor Martin Grann -- report on a total of 73 research samples comprising 24,847 people from 13 countries. Cumulatively, the samples had a high base rate of reoffense, with almost one in four reoffending over an average of about four years.

Bottom line: Risk assessment instruments are fairly good at identifying low risk individuals, but their high rates of false positives -- people falsely flagged as recidivists -- make them inappropriate “as sole determinants of detention, sentencing, and release.”

In all, about four out of ten of those individuals judged to be at moderate to high risk of future violence went on to violently offend. Prediction of sexual reoffense was even poorer, with less than one out of four of those judged to be at moderate to high risk going on to sexually offend. In samples with lower base rates, the researchers pointed out, predictive accuracy will be even poorer.

What that means, in practical terms, is that to stop one person who will go on to become violent again in the future, society must lock up at minimum one person who will NOT; for sex offenders, at least three non-recidivists must be detained for every recidivist. This, of course, is problematic from a human rights standpoint. 

Another key finding that goes against conventional wisdom was that actuarial instruments that focus on historical risk factors perform no better than tools based on clinical judgment, a finding contrary to some previous review.

The researchers included the nine most commonly used risk assessment tools, out of the many dozens that have now been developed around the world:
  • Level of Service Inventory-Revised (LSI-R) 
  • Psychopathy Checklist-Revised (PCL-R) 
  • Sex Offender Risk Appraisal Guide (SORAG) 
  • Static-99 
  • Violence Risk Appraisal Guide (VRAG) 
  • Historical, Clinical, Risk management-20 (HCR-20) 
  • Sexual Violence Risk-20 (SVR-20) 
  • Spousal Assault Risk Assessment (SARA) 
  • Structured Assessment of Violence Risk in Youth (SAVRY) 
Team leader Fazel, of Oxford University, and colleagues stressed several key implications of their findings:
One implication of these findings is that, even after 30 years of development, the view that violence, sexual, or criminal risk can be predicted in most cases is not evidence based. This message is important for the general public, media, and some administrations who may have unrealistic expectations of risk prediction for clinicians. 

A second and related implication is that these tools are not sufficient on their own for the purposes of risk assessment. In some criminal justice systems, expert testimony commonly uses scores from these instruments in a simplistic way to estimate an individual’s risk of serious repeat offending. However, our review suggests that risk assessment tools in their current form can only be used to roughly classify individuals at the group level, and not to safely determine criminal prognosis in an individual case. 

Finally, our review suggests that these instruments should be used differently. Since they had higher negative predictive values, one potential approach would be to use them to screen out low risk individuals. Researchers and policy makers could use the number safely discharged to determine the potential screening use of any particular tool, although its use could be limited for clinicians depending on the immediate and service consequences of false positives. 

A further caveat is that specificities were not high -- therefore, although the decision maker can be confident that a person is truly low risk if screened out, when someone fails to be screened out as low risk, doctors cannot be certain that this person is not low risk. In other words, many individuals assessed as being at moderate or high risk could be, in fact, low risk. 

My blog post on these researchers' previous meta-analytic study, Violence risk meta-meta: Instrument choice does matter, is HERE.