Discussion Paper: Hard Cases Make Bad Law -- Reactions to R v. T

Abstract

In the case of R v T (2010), the Court of Appeal for England and Wales rejected the testimony of an expert who had used likelihood ratios to assess the probative value of shoe-print evidence. Because likelihood ratios are widely used in forensic science, and their use has been actively promoted by leaders in the field (Association of Forensic Science Providers, 2009; Cook et al., 1998; Evett, 1998; Robertson and Vignaux, 1995), the court’s opinion has understandably caused consternation in the forensic science community (Berger et al. 2011; Robertson et al. 2011; Redmayne et al. 2011). Recognizing the importance of the issues involved, the editors of Law, Probability and Risk have devoted this issue to articles commenting on the case. At the invitation of Editor Colin Aitken, it is my privilege and honour to review and respond to those articles.

In the interest of full disclosure, I will say at the outset that I think R v T is an inept judicial opinion that creates bad law. The opinion went awry because the justices who wrote it misunderstood a key aspect of the evidence they were evaluating. The justices sought to achieve laudable goals, but their misunderstanding of basic principles of inductive logic, and particularly Bayes’ theorem, led them to exclude a type of expert evidence that, in general, is helpful and appropriate in favour of an alternative type of expert of evidence that is fundamentally inconsistent with the goals the court sought to achieve. The case has already received severe criticism1 and will inevitably come to be seen for what it is — a judicial blunder.

Keywords: law, evidence, statistics, Bayes, likelihood ratio, probability, source, forensic science, shoeprint