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Weighing Evidence in Sexual Abuse Evaluations: An Introduction to Bayes's Theorem

NCJ Number
162563
Journal
Child Maltreatment Volume: 1 Issue: 1 Dated: (February 1996) Pages: 25-36
Author(s)
J M Wood
Date Published
1996
Length
12 pages
Annotation
This article introduces readers to Bayes's theorem, a 200-year-old mathematical formula that can illuminate general issues and assist evaluators in the field of child sexual abuse.
Abstract
The theorem is applied to a case study of sexual abuse allegations that arose during a custody dispute. A woman accused her ex-husband of sexually abusing their 5-year-old son during parental visitations. However, a court-appointed psychologist determined that the mother had put sperm on her son's bottom, apparently to substantiate her allegations. The child was taken from his mother and placed in foster care pending a court decision regarding custody. Sexual abuse evaluators routinely assess the strength of evidence, using descriptors such as weak, moderate or strong. The same purpose may be accomplished by using a number called the likelihood ratio, which expresses the relative probability of coming across a particular piece of evidence in one group rather than in another. The article discusses how such an indicator is calculated and interpreted, as well as the controversy surrounding the use of indicators to evaluate sexual abuse. In the specific case studied for this article, the psychologist's opinions were accepted without disagreement by the local Child Protection Services and a judge. The child was eventually removed from his mother and placed into the permanent custody of his father. Tables, references