least squares index) is used to measure the disproportionality of an electoral outcome, that is the difference between the percentage of votes received and the percentage of seats a party gets in the resulting legislature.

electoral systems. The index involves taking the

square rootof half the

sumof the

squaresof the difference between percent of vote and percent of seats (as whole numbers) for each of the

political parties.

Michael Gallagher, who created the index, included ‘other’ parties as a whole category, and

Arend Lijphartmodified it, excluding those parties.

Loosemore–Hanby index, the Gallagher index is less sensitive to small discrepancies.