Does explaining the meaning of likelihood ratios improve lay understanding?

• Participants watched video of realistic expert testimony, including presented LRs (PLRs). • Prior and posterior odds were elicited, and effective LRs (ELRs) calculated. • ELRs were sensitive to relative differences in PLRs. • When the expert explained LRs, ELRs were slightly more likely to equal PLRs. • Explanation of LRs did not decrease the rate of occurrence of the prosecutor’s fallacy. 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.