Defence Against the Modern Arts: the Curse of Statistics -- FRStat

Abstract: For several decades, legal and scientific scholars have argued that conclusions from forensic examinations should be supported by statistical data and reported within a probabilistic framework. Multiple models have been proposed to quantify the probative value of forensic evidence. Unfortunately, several of these models rely on ad-hoc strategies that are not scientifically sound. The opacity of the technical jargon that is used to present these models and their results and the complexity of the techniques involved make it very difficult for the untrained user to separate the wheat from the chaff. This series of paper is intended to help forensic scientists and lawyers recognise issues in tools proposed to interpret the results of forensic examinations. This paper focuses on the tool proposed by the Latent Print Branch of the U.S. Defense Forensic Science Center (DFSC) and called FRStat. In this paper, I explore the compatibility of the results outputted by FRStat with the language used by the DFCS to report the conclusions of their fingerprint examinations, as well as the appropriateness of the statistical modelling underpinning the tool and the validation of its performance.