Partisan gerrymandering is always carried out by cracking a party’s supporters among many districts, in which their preferred candidates lose by relatively narrow margins; and/or by packing a party’s backers in a few districts, in which their preferred candidates win by enormous margins. Both cracking and packing produce votes that are inefficient in the sense that they do not contribute to a candidate’s election. In the case of cracking, all votes cast for the losing candidate are inefficient. In the case of packing, all votes cast for the winning candidate, above the 50% (plus one) threshold needed for victory, are inefficient. The efficiency gap is calculated by taking one party’s total inefficient votes in an election, subtracting the other party’s total inefficient votes, and dividing by the total number of votes cast. It captures in a single number the extent to which district lines crack and pack one party’s voters more than the other party’s voters.
The efficiency gap was introduced by Nicholas Stephanopoulos and Eric McGhee in their 2015 University of Chicago Law Review article, Partisan Gerrymandering and the Efficiency Gap.
Under North Carolina’s 2012-2014 congressional plan, votes for Republican candidates were inefficient at a rate 20.3% lower than votes for Democratic candidates.
Under Texas’s 1992-1994 congressional plan, votes for Democratic candidates were inefficient at a rate 20.3% lower than votes for Republican candidates.