La régionalisation électorale et l'amplification des proportions

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Abstract
In single-member constituency, simple plurality electoral systems, the disproportion between the popular vote polled by a party and the number of representatives it elects has stimulated much controversy. We review the mathematical and statistical models of two-party systems of this type which have been proposed by Kendall and Stuart, Theil, Lemieux, March, and the present authors. This leads to a detailed empirical study of series of elections in three systems: the provincial systems of Quebec and Ontario, and the Canadian federal system. In verifying a “sliding distribution” model more general than that of Kendall and Stuart, we construct the distribution of constituencies according their level of support for a given party, and hence provide an index of electoral regionalization. Less detailed seat-vote data, for longer series of elections in five systems, provide evidence for the persistence of electoral disproportions and the distinguishability of seat-vote characteristics between different systems. The extreme sensitivity of legislative proportions in Québec to small changes in popular vote proportions is shown to be a consequence not of peculiarities in the electoral map, but of the small standard deviation and large kurtosis (peakedness) of the distribution of constituencies.

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