The seats-to-votes ratio, also known as the advantage ratio, is a measure of equal representation of voters. The equation for seats-to-votes ratio for a political party i is:
where is fraction of votes and is fraction of seats.
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.
Related is the votes-per-seat-won, which is inverse to the seats-to-votes ratio.
The Sainte-Laguë Index is a disproportionality index derived by applying the Pearson's chi-squared test to the seats-to-votes ratio, the Gallagher index has a similar formula.
Different apportionment methods such as Sainte-Laguë method and D'Hondt method differ in the seats-to-votes ratio for individual parties.
The Sainte-Laguë method optimizes the seats-to-votes ratio among all parties with the least squares approach. The difference of the seats-to-votes ratio and the ideal seats-to-votes ratio for each party is squared, weighted according to the vote share of each party and summed up:
It was shown that this error is minimized by the Sainte-Laguë method.
The D'Hondt method approximates proportionality by minimizing the largest seats-to-votes ratio among all parties. The largest seats-to-votes ratio, which measures how over-represented the most over-represented party among all parties is:
The D'Hondt method minimizes the largest seats-to-votes ratio by assigning the seats,
where is a seat allocation from the set of all allowed seat allocations .