Quality and Quantity

, Volume 38, Issue 1, pp 51–74 | Cite as

Selecting Indexes of Electoral Proportionality: General Properties and Relationships

  • Galina Borisyuk
  • Colin Rallings
  • Michael Thrasher


Measuring the proportionality of outcomes in termsof each party's vote and seatshares is an important task in electoral analysis.Various indexes have been designedthat provide a summary statistic of electoralproportionality/disproportionality. Claimsand counter-claims have been made regarding thestrengths and weaknesses of particularindexes. Important consequences follow from thismethodological pluralism. First, it isnot always clear which index has been employedwhen particular electoral outcomesare discussed. Second, recent additions to thelist of indexes have not been thoroughlyscrutinised and appraised. Third, the lack ofknowledge about the general relationshipbetween indexes means that observations might bedifferent had a different index beenused. This article seeks to identify and clarifythe particular properties of different indexesof proportionality. Relatively new, and largelyuntested, indexes of proportionality areexamined and some unusual and potentially damagingproperties are identified. We alsocompare different measures of disproportionality inan effort to specify some generalproperties of the inter-relationships between them.Understanding the particular patternsof electoral competition and vote distributionsthat affect the relationship between thesemeasures should enable users to anticipate theconsequences of preferring one index overothers.

proportionality indices electoral outcome electoral disproportionality statistical measures votes seats wasted votes 


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Copyright information

© Kluwer Academic Publishers 2004

Authors and Affiliations

  • Galina Borisyuk
    • 1
  • Colin Rallings
    • 1
  • Michael Thrasher
    • 1
  1. 1.Local Government Chronicle Elections Centre, Department of PoliticsUniversity of Plymouth, Drake CircusPlymouthUK

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