The Influence of Validation Data on Logical and Scientific Interpretations of Forensic Expert Opinions

Forensic experts use specialized training and knowledge to enable other members of the judicial system to make better informed and more just decisions. Factfinders, in particular, are tasked with judging how much weight to give to experts' reports and opinions. Many references describe assessing evidential weight from the perspective of a forensic expert. Some recognize that stakeholders are each responsible for evaluating their own weight of evidence. Morris (1971, 1974, 1977) provided a general framework for recipients to update their own uncertainties after learning an expert's opinion. Although this framework is normative under Bayesian axioms and several forensic scholars advocate the use of Bayesian reasoning, few resources describe its application in forensic science. This paper addresses this gap by examining how recipients can combine principles of science and Bayesian reasoning to evaluate their own likelihood ratios for expert opinions. This exercise helps clarify how an expert's role depends on whether one envisions recipients to be logical and scientific or deferential. Illustrative examples with an expert's opinion expressed as a categorical conclusion, likelihood ratio, or range of likelihood ratios, or with likelihood ratios from multiple experts, each reveal the importance and influence of validation data for logical recipients' interpretations.