Gallagher index

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The Gallagher index (based on the method of least squares) "measures an electoral system’s relative disproportionality between votes received and seats allotted in a legislature." [1][2] 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 measures this dis-proportionality from all parties collectively in one given election. This collective dis-proportionality from this one 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 index involves taking the square root of half the sum of the squares of the difference between percent of vote () and percent of seats () for each of the political parties ().[3]


The index weighs the deviations by their own value, creating a responsive index, ranging from 0 to 100. The larger the differences between the percentage of the vote 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, who created the index, included "other" parties as a whole category, and Arend Lijphart modified it, excluding those parties. Unlike the well-known Loosemore–Hanby index, the Gallagher index is less sensitive to small discrepancies.[5]

Attention gained in Canada[edit]

The Gallagher index gained considerable attention in Canada in December 2016 in the context of efforts to reform Canada's electoral system.[6][7] The Special Committee on Electoral Reform (a Parliamentary Committee) recommended "that the Government should, as it develops a new electoral system, use the Gallagher index in order to minimize the level of distortion between the popular will of the electorate and the resultant seat allocations in Parliament." The committee recommended that "the government should seek to design a system that achieves a Gallagher score of 5 or less."[8][9] In the 2015 Canadian federal election, the Gallagher index was 12.02, where 0 would be a perfectly proportional election outcome.

2015 Canadian General Election Gallagher Index.svg

Examples of calculating disproportionality[edit]

New Zealand[edit]

This table uses the New Zealand 2005 election result.[10] Note that since New Zealand uses the MMP voting system, voters have two votes. This list uses the party vote, which determines the proportionality of the House; the electorate vote determines the local member.

party  % of votes  % of seats difference difference
Labour 41.10 41.32 0.22 0.0484
National 39.10 39.67 0.57 0.3249
NZ First 5.72 5.79 0.07 0.0049
Greens 5.30 4.96 0.34 0.1156
Māori 2.12 3.30 1.18 1.3924
United Future 2.67 2.48 0.19 0.0361
ACT 1.51 1.65 0.14 0.0196
Progressives 1.16 0.82 0.34 0.1156
Destiny 0.62 0 0.62 0.3844
Legalise Cannabis 0.25 0 0.25 0.0625
Christian Heritage 0.12 0 0.12 0.0144
Alliance 0.07 0 0.07 0.0049
Family Rights 0.05 0 0.05 0.0025
Democrats 0.05 0 0.05 0.0025
Libertarianz 0.04 0 0.04 0.0016
Direct Democracy 0.03 0 0.04 0.0016
99MP 0.03 0 0.03 0.0009
OneNZ 0.02 0 0.02 0.0004
Republicans 0.02 0 0.02 0.0004
total of squares of differences 2.5336
total / 2 1.2668
square root of (total / 2) 1.13

Thus the disproportionality of the 2005 New Zealand election is 1.13, which is very low by international standards.[11]

Note that the Māori Party has the highest difference, which is significantly above the others. This is due to New Zealand's system of reserved seats for Māori. The Māori seats are allocated by votes on a separate electoral roll, and while any party can contest these seats, they are generally won by either the Māori Party, the Labour Party, or New Zealand First.


The 2012 Election for the Australian state of Queensland resulted in a Gallagher Index of 31.16 - the winning party gained over 87% of the seats with just less than 50% of the vote

Other indices[edit]

The Sainte-Laguë method is considered by Gallagher to be "probably the soundest of all the measures."[12] This is closely related to the Pearson's chi-squared test which has better statistical underpinning.


  1. ^ Special Committee on Electoral Reform (a Canadian Parliamentary Committee) (December 1, 2016). Report 3: Strengthening Democracy in Canada : Principles, Process and Public Engagement for Electoral Reform (Report). Parliament of Canada. p. 69 (or p. 83 in PDF search). Retrieved December 26, 2016. One tool that has been developed to measure an electoral system’s relative disproportionality between votes received and seats allotted in a legislature is the Gallagher Index, which was developed by Michael Gallagher (who appeared before the Committee). 
  2. ^ O'Malley, Kady (December 1, 2016). "Read the full electoral reform committee report, plus Liberal and NDP/Green opinions". Ottawa Citizen. Ottawa Citizen. Retrieved December 26, 2016. 
  3. ^ Gallagher, Michael (1991). "Proportionality, Disproportionality and Electoral Systems". Electoral Studies. 10: 33–51. doi:10.1016/0261-3794(91)90004-c. 
  4. ^ Gallagher 1991, p. 40.
  5. ^ Gallagher 1991, p. 41.
  6. ^ Cash, Colby (December 2, 2016). "Colby Cosh: Did Maryam Monsef actually read the whole electoral reform report?". National Post. Retrieved 10 December 2016. 
  7. ^ Wherry, Aaron (December 1, 2016). "Minister 'disappointed' as electoral reform committee recommends referendum on proportional representation". CBC News. Retrieved 10 December 2016. 
  8. ^ O'Malley, Kady (December 1, 2016). "Read the full electoral reform committee report, plus Liberal and NDP/Green opinions". Ottawa Citizen. Ottawa Citizen. Retrieved 10 December 2016. 
  9. ^ "Is Canada Fair?". Measuring Unfairness — Calculating Canada's Gallagher Index. (This website includes the Gallagher Index in adjustable table format. It initially shows the data for Canada's 2015 federal election, but some variables in some table cells are adjustable by the visitor to the website, and then the rest of the table is automatically adjusted to reflect this visitor's new input.). Retrieved 10 December 2016. 
  10. ^ "Official Count Results -- Electorate Status". Chief Electoral Office- New Zealand. 1 October 2005. Retrieved 9 August 2013. 
  11. ^ Gallagher, Michael (2013). "Election indices" (PDF). Trinity College, Dublin. Retrieved 9 August 2013. 
  12. ^ Gallagher 1991, p. 47.


External links[edit]