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Efficiency gap
Wasted votes are the basis for computing the efficiency gap, a measure devised by University of Chicago law professor Nicholas Stephanopoulos and political scientist Eric McGhee in 2014. This statistic has been used to quantitatively assess the effect of gerrymandering, the assigning of voters to electoral districts in such a way as to increase the number of districts won by one political party at the expense of another. The heart of the computation is to add up, over all electoral districts, the wasted votes of each party's candidates. The efficiency gap is the difference between the two parties' wasted votes, divided by the total number of votes. Stephanopoulos and McGhee argued that in a non-partisan redistricting with two roughly equally popular parties, the efficiency gap would be zero, with an equal number of wasted votes from either party. If the gap exceeded 7%, then Stephanopoulos and McGhee argued that this could ensure the party with fewer wasted votes would be able to control the state for the duration of the validity of the district map.
Citing in part an efficiency gap of 11.69% to 13% in favor of the Republicans, a U.S. District Court ruling in Whitford v. Gill in 2016 ruled against the 2011 drawing of Wisconsin legislative districts. It was the first U.S. Federal court ruling to strike down a redistricting on the grounds of favoring a political party. In the 2012 election for the state legislature, Republican candidates had 48.6% of the two-party votes but won 61% of the 99 districts. The court found that the disparate treatment of Democratic and Republican voters violated the 1st and 14th amendments to the US Constitution. The State has challenged the District Court's ruling, and in June 2017, the Supreme Court has agreed to hear arguments in this case. The state is expected to challenge the validity of the efficiency gap measure.
