jury (theorem)

A body of people charged with deciding the truth of some claim. In democratic Athens, juries had 501, 1,001, or 1,501 members (an odd number to avoid tied votes). Then as now, jurors were selected randomly (at least in principle). The idea that it is fair that the guilt or innocence of the accused should be decided by a jury selected at random from his or her fellow-citizens was formalized by Condorcet, whose jury theorem states that if each juror is on average more likely to judge correctly than wrongly whether the accused is guilty, then the verdict of the majority will be trustworthy. In particular, each juror need be only just more likely to be right than wrong for a majority of, say, 10 to 2 to be sufficient to conclude that the majority is correct. The jury theorem is again being studied and applied in politics and other social sciences.