Efficiency Gap

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 wasted 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 wasted. In the case of packing, all votes cast for the winning candidate, above the 50% (plus one) threshold needed for victory, are wasted. The efficiency gap is calculated by taking one party’s total wasted votes in an election, subtracting the other party’s total wasted 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.

Compare the efficiency gap to two other metrics, partisan bias and the mean-median difference.

North Carolina 2012

Under North Carolina’s 2012-2014 congressional plan, votes for Republican candidates were wasted at a rate 20.3% lower than votes for Democratic candidates.

NC
+% D
Balanced
+% R

Texas 1992

Under Texas’s 1992-1994 congressional plan, votes for Democratic candidates were wasted at a rate 20.3% lower than votes for Republican candidates.

TX
+% D
Balanced
+% R