Likelihoodism, Bayesianism, and a Pair of Shoes
10 Pages Posted: 25 May 2013
Date Written: 2012
In R v. T,  EWCA (Crim) 2439, a footwear analyst followed recommendations of the Forensic Science Service in testifying to the weight of the evidence according to a standardized table for characterizing likelihood ratios, reporting that the evidence established “a moderate degree of scientific evidence to support the view that the [Nike trainers recovered from the appellant] had made the footwear marks” in question. The Court of Appeal for England and Wales offered a variety of reasons for holding that this testimony should not have been received. Although the opinion can and should be read narrowly, the apparent preference for traditional opinion testimony about the source of such trace evidence is unfortunate. This essay adds to previous criticism of that aspect of the opinion by distinguishing between likelihood and Bayesian theories of inference. It argues that courts should receptive to the efforts of forensic analysts, guided by either of these theories, to avoid source attributions and to direct their testimony to the strength of the evidence with respect to competing hypotheses.
Keywords: weight of evidence, forensic science testimony, inference, interpretation, Bayes' rule, likelihood ratio, source attrribution
JEL Classification: C10, C11
Suggested Citation: Suggested Citation