The equation for seats-to-votes ratio for a political party i is: where
is fraction of votes cast for that party and
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.
The Sainte-Laguë method optimizes the seats-to-votes ratio among all parties
Disproportionality, the difference of the parties' 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[2] 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 D'Hondt method minimizes the largest seats-to-votes ratio by assigning the seats,[5]