Evaluating partisan bias in the Texas redistricting case

Gary King, Bernard Grofman, Andrew Gelman and Jonahan Katz have submitted an amicus brief to the Supreme Court concerning the issue of redistricting in Texas. The brief was submitted on behalf of neither party:

[W]e make the case that districting plans can be evaluated with regard to partisan bias, and that such evaluation is uncontroversial in social science and does not require any knowledge or speculation about the intent of the redistricters.

The key concept, as laid out in the brief, is to identify partisan bias with deviation from symmetry. I’ll quote briefly from the brief (which is here; see also Rick Hasen’s election law blog for more links) and then mention a couple additional points which we didn’t have space there to elaborate on. I’m interested in this topic for its own sake and also because Gary and I put a lot of effort in the early 90s into figuring this stuff out).

Here’s what we wrote on partisan symmetry in the amicus brief:

The symmetry standard measures fairness in election systems, and is not specific to evaluating gerrymanders. The symmetry standard requires that the electoral system treat similarly-situated political parties equally, so that each receives the same fraction of legislative seats for a particular vote percentage as the other party would receive if it had received the same percentage. In other words, it compares how both parties would fare hypothetically if they each (in turn) had received a given percentage of the vote. The difference in how parties would fare is the “partisan bias” of the electoral system. Symmetry, however, does not require proportionality.

More, including a discussion on this criterion for evaluating bias, over at Gelman’s blog.