The partisan outcome would never have been possible

As I’ve mentioned a few times now I’m part of the campaign to end gerrymandering in Michigan (the campaign website explaining gerrymandering is here). So I’m quite interested when I find links to articles on the issue. Especially one that can satisfy my inner geek.

Nicholas Stephanopoulos, a law professor at the University of Chicago, wrote an opinion column for The Inquirer of Philadelphia. Like Michigan, Pennsylvania is highly gerrymandered. Like Michigan the voters are about evenly balanced between Dems and GOP. Yet, the GOP sent 13 representatives to Congress while the Dems sent only 5.

Back in 2004 the Supremes heard a case about gerrymandering in PA. The justices said we don’t like gerrymandering but we don’t know how to measure it. Another case, based on the 2011 redistricting, is before the PA Supremes with a ruling expected by the end of the month.

Since the 2004 case political scientists got to work. Last year I’ve discussed one measure that came out of that work, called the efficiency gap. This news article mentions a couple more. It doesn’t describe them, though does link to the scholarly papers (as PDF) that were presented in court. I took a look at the one under the link “maps” written by Jowei Chen, Ph.D, an associate professor of Political Science at the University of Michigan who has testified in several gerrymandering cases.

Chen used a computer simulation that randomly drew districts and compared the results with what the lawmakers drew. Chen lays the goals (which gives you an idea whether you want to read the rest of the paper).

By randomly drawing districting plans with a process designed to optimize on traditional districting criteria, the computer simulation process thus gives us a precise indication of the range of districting plans that plausibly and likely emerge when map-drawers are not motivated primarily by partisan goals. By comparing enacted plans against the range of simulated plans with respect to partisan measurements, I am able to determine the extent to which a map-drawer’s deviations from traditional districting criteria, such as geographical compactness and county splits, was motivated by partisan goals.

The traditional criteria for districts are: population equality (specified in the federal constitution), being contiguous, avoiding county splits, avoiding municipality splits, and geographic compactness.

Chen randomly created 500 maps using the traditional criteria. Then applied precinct voting data to each map and to the enacted map. Then he ran all his statistical measurements to the created maps and the enacted map. Finally, he drew some tables and charts.

For example, The random maps gave the GOP 7 to 10 districts, with 54% of the maps giving the GOP 9 out of 18 seats (what the overall vote tallies suggest they should get). None of the maps gave the GOP more than 10 seats. And yet the enacted map gave them 13. Chen supplied 3 more graphs of various measures. In all of them the random maps were all clustered over here and the enacted map was way over there. Chen wrote:

I thus conclude with overwhelmingly high statistical certainty that the enacted plan created a pro-Republican partisan outcome that would never have been possible under a districting process adhering to non-partisan traditional criteria.

Stephanopoulos tackles some of the arguments the GOP lawyers tried in court. In one of those arguments professor Wendy Tam Cho said that all those hundreds of simulated maps may not be representative of all possible lawful maps. Stephanopoulos says that’s true but irrelevant. All those maps do prove (1) there are hundreds of maps that are more fair, and (2) Pennsylvania geography (in which Dems tend to cluster in cities) didn’t cause the GOP to get the extra seats. The other GOP arguments are just as logically flawed.