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The opportunity prior: A simple and practical solution to the prior probability problem for legal cases

Fenton, N; Lagnado, D; Dahlman, C; Neil, M; (2017) The opportunity prior: A simple and practical solution to the prior probability problem for legal cases. In: (Proceedings) ICAIL '17 Proceedings of the 16th edition of the International Conference on Articial Intelligence and Law. (pp. pp. 69-78). Association for Computing Machinery (ACM): New York, USA. Green open access

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Abstract

One of the greatest impediments to the use of probabilistic reasoning in legal arguments is the difficulty in agreeing on an appropriate prior probability for the ultimate hypothesis, (in criminal cases this is normally “Defendant is guilty of the crime for which he/she is accused”). Even strong supporters of a Bayesian approach prefer to ignore priors and focus instead on considering only the likelihood ratio (LR) of the evidence. But the LR still requires the decision maker (be it a judge or juror during trial, or anybody helping to determine beforehand whether a case should proceed to trial) to consider their own prior; without it the LR has limited value. We show that, in a large class of cases, it is possible to arrive at a realistic prior that is also as consistent as possible with the legal notion of ‘innocent until proven guilty’. The approach can be considered as a formalisation of the ‘island problem’ whereby if it is known the crime took place on an island when n people were present, then each of the people on the island has an equal prior probability 1/n of having carried out the crime. Our prior is based on simple location and time parameters that determine both a) the crime scene/time (within which it is certain the crime took place) and b) the extended crime scene/time which is the ‘smallest’ within which it is certain the suspect was known to have been ‘closest’ in location/time to the crime scene. The method applies to cases where we assume a crime has taken place and that it was committed by one person against one other person (e.g. murder, assault, robbery). The paper considers both the practical and legal implications of the approach. We demonstrate how the opportunity prior probability is naturally incorporated into a generic Bayesian network model that allows us to integrate other evidence about the case.

Type: Proceedings paper
Title: The opportunity prior: A simple and practical solution to the prior probability problem for legal cases
Event: ICAIL '17 Proceedings of the 16th edition of the International Conference on Articial Intelligence and Law
ISBN-13: 9781450348911
Open access status: An open access version is available from UCL Discovery
DOI: 10.1145/3086512.3086519
Publisher version: https://doi.org/10.1145/3086512.3086519
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences > Div of Psychology and Lang Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences > Div of Psychology and Lang Sciences > Experimental Psychology
URI: https://discovery.ucl.ac.uk/id/eprint/10048641
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