Defining Probability in Sex Offender Risk Assessment

• Author: Richard W. Elwood
• Published date: 01 December 2016
• Date: 01 December 2016
• DOI: http://doi.org/10.1177/0306624X15587912
• Subject Matter: Articles

International Journal of

Offender Therapy and

Comparative Criminology

2016, Vol. 60(16) 1928 –1941

© The Author(s) 2015

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DOI: 10.1177/0306624X15587912

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Article

Defining Probability in Sex

Offender Risk Assessment

Richard W. Elwood1

Abstract

There is ongoing debate and confusion over using actuarial scales to predict individuals’

risk of sexual recidivism. Much of the debate comes from not distinguishing Frequentist

from Bayesian definitions of probability. Much of the confusion comes from applying

Frequentist probability to individuals’ risk. By definition, only Bayesian probability can

be applied to the single case. The Bayesian concept of probability resolves most of the

confusion and much of the debate in sex offender risk assessment. Although Bayesian

probability is well accepted in risk assessment generally, it has not been widely used

to assess the risk of sex offenders. I review the two concepts of probability and

show how the Bayesian view alone provides a coherent scheme to conceptualize

individuals’ risk of sexual recidivism.

Keywords

risk assessment, probability, sexual recidivism, prediction

Introduction

There is an ongoing debate in sex offender risk assessment over using actuarial scales

like the Static-99/R to predict the risk of individual offenders. Harris and Hanson

(2004) claimed the Static-99 assumes that “the probability of recidivism for an indi-

vidual offender will be the same as the observed recidivism rate for the group to which

he most closely belongs” (p. 9). Berlin, Galbreath, Geary, and McGlone (2003) coun-

tered this common assumption, arguing that not all individuals with the same score

share the same risk. Some writers agree and question whether the Static-99R can accu-

rately predict any individual’s risk (Cooke & Michie, 2010; Donaldson & Abbott,

2011; Donaldson, Abbott, & Michie, 2012). Others contend that the Static-99/R

1Sand Ridge Evaluation Unit, Madison, WI, USA

Corresponding Author:

Richard W. Elwood, Sand Ridge Evaluation Unit, 301 Troy Drive, Madison, WI 53704, USA.

Email: richard.elwood@wi.gov

587912IJOXXX10.1177/0306624X15587912International Journal of Offender Therapy and Comparative CriminologyElwood

research-article2015