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Neither the T index nor the D2 score measure “two-partyness”: a comment on Gaines and Taagepera

Dunleavy, Patrick ORCID: 0000-0002-2650-6398 (2014) Neither the T index nor the D2 score measure “two-partyness”: a comment on Gaines and Taagepera. Journal of Elections, Public Opinion and Parties, 24 (3). pp. 362-385. ISSN 1745-7289

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Identification Number: 10.1080/17457289.2014.902841


This comment critiques the paper by Gaines and Taagepera (2013) outlining two new measures that compare how far election outcomes diverge from a particular ideal of “perfect two-partyness” (one in which all votes are divided equally between the top two parties). Their first proposed T index is an unstable amalgam of two different measures, one linear and the other not. Applied to analysing sets of election outcomes, it systematically mis-signals “two-partyness” in its accepted meaning, producing perverse results. Their second index, D2, has a varying minimum size level depending on the size of the largest party (P1) and the number of observable parties competing. In many circumstances D2 scores bifurcate – the same scores are produced by both very low and very high P1 levels. Applied to distributions, the D2 score artefactually homogenizes very dissimilar distributions, again misreads even two-party configurations, and always overstates “two-partyness” in multi-party systems. I conclude that neither the T nor D2 indices are fit for purpose. They should not be further used in electoral analysis.

Item Type: Article
Official URL:
Additional Information: © 2014 Elections, Public Opinion & Parties.
Divisions: Government
Public Policy Group
Subjects: J Political Science > JA Political science (General)
J Political Science > JC Political theory
Date Deposited: 24 Apr 2014 15:49
Last Modified: 20 Jul 2021 02:21

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