Estimating juror accuracy, juror ability, and the relationship between them.

This article advances a method based on standard test theories and measurement models to determine correct verdicts for jury trials, and to estimate juror accuracy, juror ability, and trial difficulty (and the relationships among them). With five vignette cases and 1,318 juror eligible adults as the subjects, the model consistently identified verdicts that accorded with the judge's instructions on the law as correct. With the correct verdicts, the strength of the relationship between juror accuracy and juror ability was found to be substantial. These findings suggest that the assumption of equivalent accuracy of jurors underlying the Condorcet's jury theorem (Condorcet, Essai sur l'Application de l'Analyse a la Probabilite des Decisions Rendues a la Pluralite des Voix, Paris, 1785) may be untenable for general cases where jurors of diverse dispositions and abilities serve together; and that the role of juror ability in determining the accuracy of legal decisions could be more significant than that of attitudes and values because, unlike attitudes and values, ability could affect juror's legal decisions regardless of the type of the case.