Another look at interpreting risk categories.

Several studies over the past decade have shown that simple rating scales can accurately rank sex offenders' long-term risk of recidivism. But when using these scales as prediction tools, evaluators often wish to translate categories of risk into probabilities of recidivism. D. M. Doren (2004) has recently suggested that evaluators may use the recidivism percentages published in original studies of the RRASOR and STATIC-99 without regard to differences in populations or base rates. This article explains why Doren's computations should lead to a different conclusion, and describes how simply comparing percentages across studies can mislead researchers and clinicians. Instead, investigators should isolate and examine the detection properties of risk assessment instruments alone, independent of the population- or setting-specific base rate. This article explains this process, using an imaginary study to illustrate how base rates and the properties of risk assessment instruments yield estimated probabilities of recidivism. The article also shows why Doren's results imply that the percentages of recidivism associated with scores on the RRASOR and STATIC-99 scores may vary across study populations. The article offers recommendations for researchers who design and evaluate actuarial methods of assessing risk and for clinicians who interpret results from risk assessment instruments.