Neither the T index nor the D2 score measure "two-partyness": A comment on Gains and Taagepera

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Abstract

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.
Original languageEnglish
Pages (from-to)362-385
Number of pages24
JournalJournal of Elections, Public Opinion and Parties
Volume24
Issue number3
DOIs
Publication statusPublished - 2014
Externally publishedYes

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title = "Neither the T index nor the D2 score measure {"}two-partyness{"}: A comment on Gains and Taagepera",
abstract = "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.",
author = "Patrick DUNLEAVY",
year = "2014",
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journal = "Journal of Elections, Public Opinion and Parties",
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AU - DUNLEAVY, Patrick

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N2 - 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.

AB - 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.

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JO - Journal of Elections, Public Opinion and Parties

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