What is the error margin of your signature analysis?

In our experience, it seems to become more and more common for mandating authorities or parties to ask forensic signature examiners to quantify the degree of certainty of their conclusion regarding a signature analysis. This paper reports the likelihood ratio approach followed by examiners to answer such a question, in a case where the Court asked whether a questioned signature was written, or not, by Mr Jones. The Court also required an assessment of the error margin of the signature analysis. This question was answered using Bayes' theorem (i.e., a full Bayesian approach) and this paper seeks to show that such an approach can be used despite the popular belief that Bayes' theorem is beyond what courts may accept. Using a practical example, we present advantages of the approach we have chosen to assess our results and show that a logical approach for evidence evaluation can be followed even in a forensic discipline where no tabulated data are available. This example also illustrates a practical way of addressing the error margin question, which helps the Court understand what can be the risk of being wrong in this particular case (and not in cases in general). We further present the way these results were communicated to the fact finders in the case at hand and provide guidance as how forensic observations can logically be combined with the other elements of the case.