Important piece at the University of Pennsylvania Law Review Online by Wendy Tam Cho on the “efficiency gap,” at the centerpiece of the Wisconsin redistricting case. It begins:

The efficiency gap has recently been touted as a general partisan fairness measure with the ability to “neatly slice the Gordian knot the Court has tied for itself, explicitly replying to the Court’s ‘unanswerable question’ of ‘[h]ow much political . . . effect is too much.’” The measure was endorsed by the district court in Whitford v. Gill, and is currently on appeal to the Supreme Court. The plaintiffs in Whitford, based on a forty‐two‐year analysis of state legislative elections, have proposed a “conservative” 7% threshold on the efficiency gap as a standard for judging the constitutionality of partisan gerrymandering. If the Court adopts the efficiency gap as a partisan fairness measure, this decision could have far‐reaching implications for redistricting practices and litigation.

We examine the properties of the efficiency gap as a measure of partisan unfairness. With the right to vote, the Court has consistently sought to guard against sophisticated as well as simple‐minded modes of discrimination. We explore whether the efficiency gap is up to this task by first considering the mathematical properties of the efficiency gap. Does it provide a consistent and stable interpretation across electoral maps (for different states, for different types of elections, e.g. Congress or state houses, and for different time periods), allowing us to compare the value of partisan fairness from one map to another? We then evaluate whether the measure is impervious to the data that is used to compute it. We finally explore its ability to tap the concepts of responsiveness and bias, separate and important facets of partisan fairness.