Whitford and his fellow plaintiffs asked the Wisconsin court to use a new approach to gauge how Republican mapmakers hurt Democrats with the main tools of gerrymandering: “packing” and “cracking.” These refer to packing like-minded voters, such as supporters of the same party, into a limited number of districts or cracking their influence by scattering them across districts in numbers too small to make an impact.

University of Chicago law professor Nicholas O. Stephanopoulos and his political scientist colleague Eric McGhee, of the Public Policy Institute of California, called their theory the “efficiency gap.” Under their approach, every voter packed into a district above the threshold needed to elect a candidate from his party creates a “surplus” vote. And someone in a cracked district, who votes for a candidate that is unable to win, is a “lost” vote. Surplus and lost votes are considered wasted votes.

The efficiency gap measures the difference between the wasted votes of the two parties in an election divided by the total number of votes cast. In an ideal scenario, where individual votes have as much impact as possible, the efficiency gap would be zero. The gap in Wisconsin was 13.3 percent in 2012.

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