Likelihood Ratio as Weight of Forensic Evidence: A Metrological Perspective (1608.07598v2)

Abstract: In this article we provide a rebuttal against the possible perception that a single number, such as the Likelihood Ratio, can provide an objective, authoritative or definitive weight of evidence. We also illustrate the extent to which conclusions can vary depending on the assumptions used in the analysis, even under alternative assumptions that are judged to be consistent with available empirical information. To facilitate these goals, we introduce the notion of a Lattice of Assumptions and an Uncertainty Pyramid illustrated in the context of a previously published example involving glass evidence. We take the position that rather than focusing on a single number summary as the weight of evidence it is the duty of the forensic expert to assist the trier of fact in forming their own interpretations from a clear understanding of the objective and demonstrably available information. We hope the presented arguments will inspire those in the forensic science community to pursue establishing their practice on a solid foundation of measurement science.