Compare methods

	Gallagher Disproportionality Index	Electoral System Analysis
Field	Political Economy	Political Economy
Family	Process / pipeline	Process / pipeline
Year of origin	1991	1954
Originator	Michael Gallagher	Maurice Duverger; Arend Lijphart; Rein Taagepera & Matthew Shugart
Type	Descriptive index of electoral outcomes	Comparative institutional analysis framework
Seminal source	Gallagher, M. (1991). Proportionality, Disproportionality and Electoral Systems. Electoral Studies, 10(1), 33-51.	Duverger, M. (1954). Political Parties: Their Organization and Activity in the Modern State. Wiley. ISBN: 9780416683202
Aliases	Gallagher Index, Least Squares Index, LSq Index, Electoral Disproportionality Index	Electoral Systems Analysis, Analysis of Electoral Rules, Votes-to-Seats Analysis
Related	2	3
Summary	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.	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.