Bayes and the Law Norman Fenton, Martin Neil and Daniel Berger
[email protected] January 2016 This is a pre-publication version of the following article: Fenton N.E, Neil M, Berger D, “Bayes and the Law”, Annual Review of Statistics and Its Application, Volume 3, 2016, doi: 10.1146/annurev-statistics-041715-033428 Posted with permission from the Annual Review of Statistics and Its Application, Volume 3 (c) 2016 by Annual Reviews, http://www.annualreviews.org. Abstract Although the last forty years has seen considerable growth in the use of statistics in legal proceedings, it is primarily classical statistical methods rather than Bayesian methods that have been used. Yet the Bayesian approach avoids many of the problems of classical statistics and is also well suited to a broader range of problems. This paper reviews the potential and actual use of Bayes in the law and explains the main reasons for its lack of impact on legal practice. These include misconceptions by the legal community about Bayes’ theorem, over-reliance on the use of the likelihood ratio and the lack of adoption of modern computational methods. We argue that Bayesian Networks (BNs), which automatically produce the necessary Bayesian calculations, provide an opportunity to address most concerns about using Bayes in the law. Keywords: Bayes, Bayesian networks, statistics in court, legal arguments 1 1 Introduction The use of statistics in legal proceedings (both criminal and civil) has a long, but not terribly well distinguished, history that has been very well documented in (Finkelstein, 2009; Gastwirth, 2000; Kadane, 2008; Koehler, 1992; Vosk and Emery, 2014).