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Bayesian Inference and Forensic Science: Problems and Perspectives

1987; Wiley; Volume: 36; Issue: 2/3 Linguagem: Inglês

10.2307/2348502

2517-6153

I.W. Evett,

Anomaly Detection Techniques and Applications

The debate about whether or not Bayesian inference provides a model for the process of assessing evidence in a court of law has generated a good amount of literature and some strongly voiced opinions. Let me start by making my own position clear: I am a forensic scientist and I am interested in the modelling of the legal process only to the extent that it enables the role of the scientist's evidence to be defined. The broader arguments centre on concepts such as the odds on guilt, but my professional colleagues jealously guard their detachment from the deliberations of guilt or otherwise and, personally, I am reluctant to handle equations which contain guilt probabilities. I shall concentrate on the problem of evaluating forensic science evidence against two narrower alternatives: C, the event that the suspect was at the crime scene and C, the event that the suspect was not at the crime scene. The great advantage of Bayesian inference is that it enables us to identify and, in principle, to answer the most appropriate range of questions which the scientist should address to be of greatest assistance to the investigator or to the court. Bayes Theorem shows us that, while the investigator or court is concerned with questions of the type: what is the probability that the suspect was at the crime scene?, the scientist, through the likelihood ratio, should address questions of the type what is the probability of the evidence given that the suspect was at the crime scene? and what is the probability of the evidence given that the suspect was not at the crime scene?. While this might appear almost self-evident to a practising Bayesian, it is only recently that forensic scientists have begun to be converted. Nevertheless there is now a growing realisation that Bayesian methods have something to offer and I am optimistic that over the coming years we can do a lot to fan that small flame. I propose next to describe the essentials of the forensic science transfer problem; then to describe briefly some of the work that has been done so far on developing solutions; then to look at ways in which developments could take place from here, emphasising the major challenges that are to be faced.