Gallagher-Index

The Gallagher index (or indices of least squares ) is a method to determine the disproportionality of an election result, which results in the difference of the voting rights held by a party in the election and the percentage of seats the party in the next parliamentary term. This involves taking the square root of half the sum of the square of the difference between voting power and mandate proportion for each of the political parties.

Lsq stands for least square, so the smallest square. n indicates the number of parties, V stands for votes ( votes) and S for seats ( seats in parliament ).

The index weighted deviations according to their own value, creating a responsive index is created, ranging from 0 to 100. The smaller the index, the lower the disproportion and vice versa. Michael Gallagher, who set the index closed other parties as a separate category with one. Arend Lijphart modified the Index by precluded this.

The index was developed in 1989 and 1991 for the first time in the article Proportionality, Disproportionality and Electoral Systems. published in the journal Electoral Studies. According to Gallagher, the index based on the work of today's electoral system scientist and an article by André Sainte- Laguë from the year 1911.

Calculation using the example of New Zealand

The table uses the results of the 2005 election in New Zealand. The New Zealand voters have two votes, this table indicates the party votes, which determines the proportionality of the House of Representatives. The second vote determines the local members. (see also the parties of New Zealand )

Accordingly, the disproportionality of choice in New Zealand in 2005 was 1.11, which represents a very small value. The Māori Party has it the largest difference, this is significantly higher than the other. This is due to the New Zealand system of Māori Electorates.

Values ​​for Germany

In a document Gallaghers, which describes the index values ​​for more than 900 elections in over 90 countries, and Germany is represented with 26 elections. The highest Gallagher index was 4.69 while in the general election before 1957, followed by the general election in 1990 with 4.63 and the general election in 2002 with 4.61. The lowest values ​​are 0.50 in the general election in 1983, 0.59 in the general election in 1976 and 0.67 in the general election in 1972. Gallagher says he did CDU and CSU considered as individual parties, when viewed as CDU/CSU- parliamentary Group would incur different values ​​for n.

Other indexes

The Gallagher index is the Loosemore - Hanby index similar to that calculates the sum of the absolute difference between the percentage and number of seats.

The Gallagher index is more sensitive to large deviations between the percentage and number of seats.

The Sainte- Laguë index is considered by Gallagher as "probably the most solid of all measurements ."

Here there is a strong proximity to Pearson's Chi -square test, and this has a better statistical support.

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