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	Gallagher Disproportionality Index	Effective Number of Parties
Field	Political Economy	Political Economy
Family	Process / pipeline	Process / pipeline
Year of origin	1991	1979
Originator	Michael Gallagher	Markku Laakso & Rein Taagepera
Type	Descriptive index of electoral outcomes	Descriptive index of party-system fragmentation
Seminal source	Gallagher, M. (1991). Proportionality, Disproportionality and Electoral Systems. Electoral Studies, 10(1), 33-51.	Laakso, M., & Taagepera, R. (1979). Effective Number of Parties: A Measure with Application to West Europe. Comparative Political Studies, 12(1), 3-27.
Aliases	Gallagher Index, Least Squares Index, LSq Index, Electoral Disproportionality Index	Laakso-Taagepera Index, ENEP and ENPP, Effective Number of Parties Index, ENP
Related	2	2
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.	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.