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Today, I filed a friend-of-the-court brief in the Harris v. Arizona Independent Redistricting Commission case (S. Ct. 14-232). The brief can be found here (for a summary of other briefs, see the Arizona Eagletarian blog). In it, I argue that the Supreme Court should reject Harris’s case on the grounds that there was no partisan injury.

Claims of partisan gerrymandering are sometimes wielded fairly loosely. Harris et al. claim that voters of one party (in their case, Republicans) were packed into districts by the Commission to impair their representation. My brief describes how such a claim can be put to an impartial mathematical test. In fact, there was no partisan asymmetry to what the Commission did. Ironically, my analysis finds a slight (but statistically nonsignificant) partisan tilt favoring Republicans, who are the complainants in this case.

The test I proposed is a simple one: the difference between the average support for a party in a state, and the median district-by-district outcome. The average-median difference* has well-known statistical properties, and can be applied to any statewide districting scheme. It might be useful in the future as a general standard for statewide partisan gerrymandering. A need for such a standard for partisan asymmetry has been expressed in the Vieth v. Jubelirer and LULAC v. Perry cases.

I have written more generally on rigorous measures of partisan symmetry. A current version of my article, “A Three-Prong Standard for Partisan Gerrymandering,” is available at SSRN.

* The average-median difference has also been proposed by Michael McDonald (SUNY Binghamton) and collaborators. My own contribution is to point out that this measure, which was formulated around 1897 by the pioneering statistician Karl Pearson, has well-behaved statistical properties which can be used easily by a court.