Compare methods

	Effective Number of Parties	Electoral System Analysis	Gallagher Disproportionality Index
Field	Political Economy	Political Economy	Political Economy
Family	Process / pipeline	Process / pipeline	Process / pipeline
Year of origin	1979	1954	1991
Originator	Markku Laakso & Rein Taagepera	Maurice Duverger; Arend Lijphart; Rein Taagepera & Matthew Shugart	Michael Gallagher
Type	Descriptive index of party-system fragmentation	Comparative institutional analysis framework	Descriptive index of electoral outcomes
Seminal source	Laakso, M., & Taagepera, R. (1979). Effective Number of Parties: A Measure with Application to West Europe. Comparative Political Studies, 12(1), 3-27.	Duverger, M. (1954). Political Parties: Their Organization and Activity in the Modern State. Wiley. ISBN: 9780416683202	Gallagher, M. (1991). Proportionality, Disproportionality and Electoral Systems. Electoral Studies, 10(1), 33-51.
Aliases	Laakso-Taagepera Index, ENEP and ENPP, Effective Number of Parties Index, ENP	Electoral Systems Analysis, Analysis of Electoral Rules, Votes-to-Seats Analysis	Gallagher Index, Least Squares Index, LSq Index, Electoral Disproportionality Index
Related	2	3	2
Summary	The effective number of parties is the standard measure of party-system fragmentation, introduced by Markku Laakso and Rein Taagepera in 1979. Rather than simply counting how many parties exist, it weights each party by its relative size, so that a handful of dominant parties count for more than a long tail of negligible ones. Formally it is the reciprocal of the Herfindahl concentration of party shares: N equals one divided by the sum of squared shares. Computed on vote shares it yields the effective number of electoral parties (ENEP); computed on seat shares it yields the effective number of parliamentary parties (ENPP). The index gives a single, intuitive number — roughly the count of equally sized parties that would produce the observed concentration — and is the workhorse for describing and comparing party systems.	Electoral system analysis examines how the rules for translating votes into seats shape the proportionality of representation and the format of the party system. Maurice Duverger's 1954 work established the foundational propositions — that single-member plurality tends toward two-party competition while proportional representation favors multipartism — through the joint operation of a mechanical effect (how the seat-allocation rule itself distorts the vote) and a psychological effect (how voters and elites anticipate that rule and behave strategically). Rein Taagepera and Matthew Shugart (1989) put the field on a quantitative footing by showing how district magnitude and other parameters systematically determine outcomes, and Arend Lijphart (1994) provided the major comparative study of disproportionality and party systems across democracies. The method reads electoral rules as a powerful, designed lever over the structure of political competition.	The Gallagher disproportionality index, also called the least squares index (LSq), is the standard summary measure of how faithfully an electoral system translates votes into seats. Introduced by Michael Gallagher in 1991, it takes the difference between each party's vote share and its seat share, squares those differences, sums and halves them, and takes the square root. Because deviations are squared before aggregation, the index gives disproportionate weight to a few large discrepancies rather than many small ones, capturing the intuition that one badly over- or under-represented party distorts the result more than scattered rounding errors. It has become the most widely reported single-number diagnostic of electoral-system performance in comparative political economy.