Sexual Abuse: A Journal of Research and Treatment, Vol. 18, No. 1, January 2006 (
Another Look at Interpreting Risk Categories
Published online: 26 April 2006
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. 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.
KEY WORDS: actuarial; risk; sex offender; recidivism; likelihood ratio; Bayes’s Theorem.
Over the past decade, investigators working in a broad variety of contexts
have established that simple instruments for evaluating levels risk—often called
Division of Forensic Psychiatry, Boonshoft School of Medicine, Wright State University, Dayton,
Glenn M. Weaver Institute of Law and Psychiatry, University of Cincinnati College of Law, Cincinnati,
To whom correspondence should be addressed at East Medical Plaza, First Floor, 627 S. Edwin C.
Moses Boulevard, Dayton, Ohio 45408-1461; e-mail: firstname.lastname@example.org.
2006 Springer Science+Business Media, Inc.