Two Fagan nomograms for the (genetic) evidence in a judicial context

While there are many computer programs that derive the likelihood ratio for genetic and other types of evidence, the Bayesian method of combination of prior probability coupled with the likelihood ratio, used by courts to ascertain posterior probability, has less support from software and other practical aids. Without proper instructions to a court, Bayes’ rule is either applied subconsciously by the judge, or prior probability is, by default, assigned a flat 50% probability by a forensic expert. In the worst case, it can be interpreted wrongly. A graphical representation using a nomogram is a proven way of representing Bayes’ theorem but remains underused in the field of forensics and the law. For this paper, I used the freeware package PyNomo to prepare two simple forensic nomograms allowing the graphical calculation of posterior probability by increasing or decreasing the prior probability due to evidence.