Some observations on the use of probability scales in forensic identification

Abstract

The increasing prominence of DNA technology as an evidential tool, coupled with the growing interest in the application of Bayesian statistics that has followed in its wake, is making itself increasingly felt in many fields of forensic expertise. It has given fresh impetus to the ongoing discussion about the expression of conclusions in those areas where interpretation of the findings plays an important role, as in handwriting analysis and speaker identification. Recent casework is used to illustrate the inadequacy of some of the present scales and for areas such as writer and speaker identification a modified probability scale is advocated to replace them. The implications of the continued use of 'logically incorrect' conclusions -- those which address the probability of a hypothesis given the evidence rather than the probability of the evidence given a hypothesis -- are examined with particular reference to handwriting and speaker identification.