What is the efficiency gap, and how is it used in redistricting analysis?

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The efficiency gap is a way to quantify how many votes are effectively wasted in a set of districts to reveal possible partisan advantage in redistricting. In each district, all votes for the losing candidate are wasted, and in districts won by a party, only the minimum number of votes needed to win (about half plus one) are not wasted—the rest are wasted. By summing wasted votes for each party across all districts and comparing the totals, then dividing by the total number of votes cast, you get the efficiency gap as a percentage. A larger gap signals a greater potential for partisan bias in the map, helping analysts assess whether a redistricting plan unfairly benefits one party. This metric is specifically about how district boundaries translate votes into seats; it isn’t about voter turnout, ballot-counting speed, or fundraising efficiency.