Another Look at Interpreting Risk Categories

Abstract

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.

Development of Vermont Assessment of Sex Offender Risk-2 (VASOR-2) Reoffense Risk Scale

“…Additionally, as Mossman (2006) cautions, even among jurisdictions with similar sexual recidivism base rates, absolute recidivism rates associated with a relative risk categories can still differ significantly.…”

Sexually Violent Predators: Toward Reasonable Estimates of Recidivism Base Rates

“…For his purposes, Dr. Tist assumes the classification accuracy of these procedures remain generally stable at each hurdle and across base rates (Frederick & Bowden, , ; Glaros & Kline, ; McFall, ; cf. Mossman, ; Singh, ). He is aware that a procedure's sensitivity provides the percentage of recidivists who are correctly classified as recidivists; that is, sensitivity provides the true positive rate.…”

Sex offender risk assessment: Consideration of human rights in community protection legislation

“…Rates of recidivism associated with groups of offenders, and the statistical properties of a risk measure, do not tell us an individual offender's probability of recidivism. Rather, this information tells us how a scale or measure performed in differentiating between recidivists and non‐recidivists in a given study or set of studies (Mossman, 2006). In the interest of protecting the human rights of the offender, the strengths as well as the substantial limitations of current risk assessment measures must be made as clear as possible to those making decisions about the individual's freedom.…”