Measuring How Far Seats Drift from Votes

Every electoral system promises to turn votes into seats, but almost none of them do it perfectly. A party can win 14 percent of the national vote and end up with far less than 14 percent of the seats, or sometimes more. Political scientists need a way to measure that drift because eyeballing a list of parties and their separate gaps does not tell you whether one country's elections are more distorted than another's. A single number lets you rank elections and systems against each other.

The basic building block is the seat-vote gap: a party's share of seats minus its share of votes. List every party's gap, and you have a mess of positive and negative numbers that do not summarize anything. A disproportionality index solves this by combining all those gaps into one score for the whole election, usually by squaring or summing the differences so they cannot cancel each other out. A higher score means seats are distributed very differently from how people actually voted. A score near zero means the chamber looks almost exactly like the vote.

This measurement sits right before the comparison between plurality and proportional representation systems. Plurality systems, like single-member districts where the top vote-getter wins everything, tend to produce high disproportionality because small parties with scattered support win few or no seats despite real vote totals. Proportional systems are built to keep that index low, since seats are handed out to match vote shares closely. Measuring disproportionality is what lets you actually prove that claim with numbers instead of just asserting it.