  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.
