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Published online by Cambridge University Press: 05 June 2014
Paris, 14 August 1835 Gentlemen, what do you think is the probability of a jury decision, in which the majority is seven against five? Without a doubt, you will be shocked at the result. You will find that the probability of error is about one in four.
Oh! Oh! Laughter from the left
I shall assert that in a large number of jury decisions, given by a majority of eight to four, an eighth are marred by error – of eight who mount the scaffold, there is on average one who is innocent.
Loud denials from the centre. Long agitation
Such, gentlemen, are the results furnished by the calculus of probabilities, and provide the data needed to resolve our question.
Renewed agitation … the speaker is interrupted … private conversations break out on every bench
Here is a way in which the new statistics seemed to matter. In 1785 Condorcet applied probability theory to judicial questions. In 1815 Laplace made some powerful a priori deductions about conviction rates. Once judicial statistics were available, his protégé Poisson used statistical inferences to overturn his conclusions. There is then a simple three-stage story of probability arithmetic and the French jury. To repeat:
1785: no jury, no experience, no data. Condorcet deduced that the optimum twelve man jury will be one that can convict with a majority of ten or more members. But he preferred a jury of 30. […]
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