A formula goes to court: Partisan gerrymandering and the efficiency gap

Abstract: Recently, a proposal has been advanced to detect unconstitutional partisan gerrymandering with a simple formula called the efficiency gap. The efficiency gap is now working its way towards a possible landmark case in the Supreme Court. This note explores some of its mathematical properties in light of the fact that it reduces to a straight proportional comparison of votes to seats. Though we offer several critiques, we assess that EG can still be a useful component of a courtroom analysis. But a famous formula can take on a life of its own and this one will need to be watched closely.