Gallagher-Index

The Gallagher Index (or least squares) can be used to measure the disproportionality of electoral systems, in particular majoritarian 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.

$LSq={\sqrt {{\frac {1}{2}}\Sigma (V-S)^{2}}}$

The Gallagher Index weighs the deviations by their own value, creating a responsive index. Martin Gallagher, who originally created the index, included 'other' parties as a whole category, Arend Lijphart modified it and excluded those parties.