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Thompson W.C., Grady R.H., Morrison G.S. (2025). Does explaining the meaning of likelihood ratios improve lay understanding? Manuscript submitted for publication

Most previous research exploring the understanding of likelihood ratios by laypersons has not provided participants with an explanation of the meaning of likelihood ratios. Although triers of fact in common-law jurisdictions usually receive oral testimony, most previous research has presented experiments in written format. The research reported in this paper presented participants with videoed testimony and tested the effect of the expert witness providing participants with an explanation of the meaning of likelihood ratios. Analysis included comparing each participant’s effective likelihood ratio (the posterior odds elicited from the participant divided by the prior odd elicited from the participant) with the presented likelihood ratio. The percentage of participants whose effective likelihood ratios equalled the presented likelihood ratios was higher for participants who were provided with the explanation of the meaning of likelihood ratios than for participants who were not provided with the explanation. The difference was, however, small. The percentage of participants whose posterior odds were consistent with them having committed the prosecutor’s fallacy was not lower for participants who were provided with the explanation of the meaning of likelihood ratios than for participants who were not provided with the explanation. The full set of results do not constitute convincing evidence that presenting the explanation of the meaning of likelihood ratios resulted in better understanding of likelihood ratios. We discuss whether there are factors other than participants not understanding the meaning of likelihood ratios that could have contributed to the results.

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Morrison G.S., Bali A.S., Martire K.A., Grady R.H., Thompson W.C. (2025). What is the best way to present likelihood ratios? A review of past research and recommendations for future research. Science & Justice, Manuscript accepted for publication.

As a first step in addressing the research question “What is the best way for forensic practitioners to present likelihood ratios so as to maximize their understandability for legal-decision makers?”, this paper reviews existing empirical literature on the comprehension of likelihood ratios by laypersons. The existing literature tends to research understanding of expressions of strength of evidence in general, rather than focusing specifically on likelihood ratios. We review the literature with respect to the CASOC indicators of comprehension (particularly sensitivity, orthodoxy, and coherence), and compare different formats that have been used to express likelihood ratios: numerical likelihood-ratios values, numerical random-match probabilities, and verbal strength-of-support statements (none of the studies that we reviewed tested comprehension of verbal likelihood ratios). We also critically review the studies with respect to methodology, and consider additional factors that could potentially assist with communication of the meaning of likelihood ratios. We conclude that the existing literature does not answer our research question, but, based on our review, we provide recommendations for the methodology of future research aimed at addressing our research question.

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