On the Significance of the Absolute Margin
AbstractConsider the hypothesis H that a defendant is guilty (a patient has condition C), and the evidence E that a majority of h out of n independent jurors (diagnostic tests) have voted for H and a minority of k:=n-h against H. How likely is the majority verdict to be correct? By a formula of Condorcet, the probability that H is true given E depends only on each juror’s competence and on the absolute margin between the majority and the minority h-k, but neither on the number n, nor on the proportion h/n. This paper reassesses that result and explores its implications. First, using the classical Condorcet jury model, I derive a more general version of Condorcet’s formula, confirming the significance of the absolute margin, but showing that the probability that H is true given E depends also on an additional parameter: the prior probability that H is true. Second, I show that a related result holds when we consider not the degree of belief we attach to H given E, but the degree of support E gives to H. Third, I address the implications for the definition of special majority voting, a procedure used to capture the asymmetry between false positive and false negative decisions. I argue that the standard definition of special majority voting in terms of a required proportion of the jury is epistemically questionable, and that the classical Condorcet jury model leads to an alternative definition in terms of a required absolute margin between the majority and the minority. Finally, I show that the results on the significance of the absolute margin can be resisted if the so-called assumption of symmetrical juror competence is relaxed.
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Bibliographic InfoPaper provided by EconWPA in its series Public Economics with number 0211004.
Length: 22 pages
Date of creation: 18 Nov 2002
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Note: Type of Document - PDF; prepared on Windows; pages: 22. This paper is included in the Nuffield College Working Paper Series in Politics at http://www.nuff.ox.ac.uk/Politics/papers/
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Condorcet jury theorem; Bayes's theorem; voting; epistemic justification; hypothesis testing;
Find related papers by JEL classification:
- D71 - Microeconomics - - Analysis of Collective Decision-Making - - - Social Choice; Clubs; Committees; Associations
- D72 - Microeconomics - - Analysis of Collective Decision-Making - - - Political Processes: Rent-seeking, Lobbying, Elections, Legislatures, and Voting Behavior
This paper has been announced in the following NEP Reports:
- NEP-ALL-2002-11-28 (All new papers)
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"A Model of Jury Decisions Where All Jurors Have the Same Evidence,"
2002-W23, Economics Group, Nuffield College, University of Oxford.
- Christian List & Franz Dietrich, 2002. "A Model of Jury Decisions Where All Jurors Have The Same Evidence," Economics Series Working Papers 2002-W23, University of Oxford, Department of Economics.
- Kanazawa, Satoshi, 1998. "A brief note on a further refinement of the Condorcet Jury Theorem for heterogeneous groups," Mathematical Social Sciences, Elsevier, vol. 35(1), pages 69-73, January.
- Christian List, 2003. "What is special about the proportion? A research report on special majority voting and the classical Condorcet jury theorem," Public Economics 0304004, EconWPA.
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