The Laakso-Taagepera Index in A Mean and Variance Framework

Scott L. Feld and Bernard Grofman
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Scott L. Feld: Department of Sociology & Anthropology, Purdue University, 700 West State Street, West Lafayette, IN 47907, USA
Bernard Grofman: Department of Political Science, University of California, Irvine, 3151 Social Science Plaza, Irvine, CA 92697-5100, USA; bgrofman@uci.edu

Journal of Theoretical Politics, 2007, vol. 19, issue 1, 101-106

Abstract: The Laakso-Taagepera index (Laakso and Taagepera, 1979) has become the most commonly used measure to specify the ‘effective’number of political parties in a party system where parties vary substantially in their vote and/or seat shares. It is well known that the Laakso-Taagepera index is the inverse of the even more widely used Herfindahl-Hirschman index of concentration (Hirschman, 1945; Herfindahl, 1950; cf. Taagepera and Grofman, 1981). Drawing on little known work by Feld and Grofman (1977, 1980) on the so called ‘class-size paradox’, it can also be shown that both indices may be re-expressed as simple functions of a distribution's mean and variance. As far as we can judge, these latter relationships appear to be unknown in the party and electoral systems literatures. By expressing the Herfindahl-Hirschman index and the Laakso-Taagepera index in terms of means and variances we can see that each index has a ‘natural’ interpretation in terms of well known statistical parameters which allows their fundamental mathematical properties to be more clearly revealed.

Keywords: electoral rules; Laakso-Taagepera Index; proportionality; seats and votes; voting (search for similar items in EconPapers)
Date: 2007
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Citations: View citations in EconPapers (2)

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Persistent link: https://EconPapers.repec.org/RePEc:sae:jothpo:v:19:y:2007:i:1:p:101-106

DOI: 10.1177/0951629807071021

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