How to Operationalize Two-Partyness

Brian J Gaines,Rein Taagepera

doi:10.1080/17457289.2013.770398

Abstract

Because of the prominence of Duverger's law, a great deal of work has focused on assessing whether or not given election outcomes can be regarded as exhibiting two-party competition. The most common metric for assessment is whether the effective number of parties is close to two. We introduce two statistics that better measure conformity to two-partyness, briefly survey their logical properties, and then demonstrate their utility with data on parliamentary elections in England.

Highlights

“The simple majority single ballot system favors the two-party system” according to a claim that, almost uniquely in political science, is dubbed a “law” (Duverger, 1954: 217)
We introduce two alternative statistics that seem preferable on logical grounds, and, incidentally, seem better to match intuition of experts
Given the popularity of trying to distinguish two- from one- and more-than-two-party competition, statistics designed expressly to measure that criterion represent a useful advance just the same

Summary

Introduction

“The simple majority single ballot system favors the two-party system” according to a claim that, almost uniquely in political science, is dubbed a “law” (Duverger, 1954: 217). The Laakso-Taagepera (1979) effective number of parties (N) is defined as the inverse of the sum of squared fractional shares of components, such as votes or seats.

Results

Conclusion