Gallagher index

The Gallagher index measures an electoral system's relative disproportionality between votes received and seats in a legislature. As such, it measures the difference between the percentage of votes each party gets and the percentage of seats each party gets in the resulting legislature, and it also measures this disproportionality from all parties collectively in any one given election. That collective disproportionality from the election is given a precise score, which can then be used in comparing various levels of proportionality among various elections from various electoral systems. The Gallagher index is a statistical analysis methodology utilised within political science, notably the branch of psephology. Michael Gallagher, who created the index, referred to it as a "least squares index", inspired by the sum of squares of residuals used in the method of least squares. The index is therefore commonly abbreviated as "LSq" even though the measured allocation is not necessarily a least squares fit. The Gallagher index is computed by taking the square root of half the sum of the squares of the difference between percent of votes () and percent of seats () for each of the political parties (). The division by 2 gives an index whose values range between 0 and 100. The larger the differences between the percentage of the votes and the percentage of seats summed over all parties, the larger the Gallagher index. The larger the index value the larger the disproportionality and vice versa. Michael Gallagher included "other" parties as a whole category, and Arend Lijphart modified it, excluding those parties. Unlike the Loosemore–Hanby index, the Gallagher index is less sensitive to small discrepancies. Other indices measuring the proportionality between seat share and party vote share are the Loosemore–Hanby index, Rae index and the Sainte-Laguë Index. The Gallagher index gained considerable attention in Canada in December 2016 in the context of efforts to reform Canada's electoral system.

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Related concepts (5)

Comparison of electoral systems

Comparison of electoral systems is the result of comparative politics for electoral systems. Electoral systems are the rules for conducting elections, a main component of which is the algorithm for determining the winner (or several winners) from the ballots cast. This article discusses methods and results of comparing different electoral systems, both those that elect a unique candidate in a 'single-winner' election and those that elect a group of representatives in a multiwinner election.

Electoral system

An electoral system or voting system is a set of rules that determine how elections and referendums are conducted and how their results are determined. Electoral systems are used in politics to elect governments, while non-political elections may take place in business, non-profit organisations and informal organisations. These rules govern all aspects of the voting process: when elections occur, who is allowed to vote, who can stand as a candidate, how ballots are marked and cast, how the ballots are counted, how votes translate into the election outcome, limits on campaign spending, and other factors that can affect the result.

One man, one vote

"One man, one vote", or "one person, one vote", expresses the principle of equal representation in voting. This slogan is used by advocates of democracy and political equality, especially with regard to electoral reforms like universal suffrage and proportional representation. The violation of equal representation in the various systems of proportional representation can be measured with the Loosemore–Hanby index, the Gallagher index, or the amount of wasted vote.