Seats-to-votes ratio

The seats-to-votes ratio,^[1] also known as the advantage ratio,^[2] is a measure of equal representation of voters. The equation for seats-to-votes ratio for a political party i is:

${\displaystyle \mathrm {a_{i}} =s_{i}/v_{i}}$,

where ${\displaystyle \mathrm {v_{i}} }$ is fraction of votes cast for that party and ${\displaystyle s_{i}}$ is fraction of seats won by that party.

In the case both seats and votes are represented as fractions or percentages, then every voter has equal representation if the seats-to-votes ratio is 1. The principle of equal representation is expressed in slogan one man, one vote and relates to proportional representation. The seats-to-votes ratio is used as the basis for the Gallagher index method of analyzing proportionality or disproportionality.

Related is the votes-per-seat-won,^[3] which is inverse to the seats-to-votes ratio.

Also related are the principles of one man one vote and representation by population.