The interpretation of conventional and 'Bayesian' verbal scales for expressing expert opinion: a small experiment among jurists

Abstract

The Bayesian framework is a useful logical concept which potentially applies to a large number of areas in forensic science, including speaker identification. According to this line of reasoning, the forensic expert is not in a position to draw conclusions about the probability of a hypothesis, e.g. the probability that the unknown speaker on a tape recording is the suspect, unless the expert is absolutely sure. If there is any doubt, the expert can only say how likely it is for the similarities and differences between the voice on the tape and the suspect's voice to occur if the suspect is indeed the speaker on the tape and if he/she is not. The Bayesian approach also implies that conventional verbal scales for expressing probability conclusions are logically incorrect. The Netherlands Forensic Institute is therefore considering introducing an alternative scale. We will report on a small-scale experiment among jurists on the interpretation of such scales.