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Avoiding the Transposed Conditional

NCJ Number
162684
Journal
Science and Justice Volume: 35 Issue: 2 Dated: (April-June 1995) Pages: 127-131
Author(s)
I W Evett
Date Published
1995
Length
5 pages
Annotation
The need for forensic scientists to avoid transposing the conditional (making probability statements that are logically incorrect) in court is discussed.
Abstract
A probability statement has little meaning unless it includes at least some indication of the information, knowledge, and assumptions upon which it is based. However, the education of the majority of scientists fails to achieve an appreciation of the nature of probability. Although it may be comparatively easy to note correct and incorrect written sentences, it becomes more difficult with the spoken word. Two sentences exemplify the issue of transposing the conditional: (1) the probability that an animal has four legs if it is a cow is one does not mean the same thing as: (2) the probability that an animal is a cow if it has four legs is one. Experience at court reveals that questions from attorneys and judges are often wrongly framed. Questions from attorneys are often phrased as transposed conditionals. Scientific expert witnesses should therefore be aware of the logical steps they are following and must use both pragmatism and common sense in determining when to give an opinion regarding identification. Case examples and 6 references (Author abstract modified)

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