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. Least squares is a mathematical optimization technique that attempts to find a best fit to a set of data by attempting to minimize the sum of the squares of the differences (called residuals) between the fitted function and the data. ... Majoritarianism is a political philosophy or agenda which asserts that a majority (sometimes categorized by religion, language or some other identifying factor) of the population is entitled to the unfettered and unbounded exercise of majority rule within a democratic or republican government—e. ... In mathematics, the principal square root of a non-negative real number is denoted and represents the non-negative real number whose square (the result of multiplying the number by itself) is . ... Addition is one of the basic operations of arithmetic. ... In algebra, the square of x is written x2 and is defined as the product of x with itself: x × x. ... A political party is a political organization that subscribes to a certain ideology and seeks to attain political power within a government. ...

The Gallagher Index weighs the deviations by their own value, creating a responsive index. The lower the index value of a country is the lower the amount of disproportionality is in this country and vice versa. Michael Gallagher, who originally created the index, included 'other' parties as a whole category, Arend Lijphart modified it and excluded those parties.