Gallagher Index

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." 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 (${\displaystyle V_{i}}$) and percent of seats (${\displaystyle S_{i}}$) for each of the political parties (${\displaystyle i=1...n}$).

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