Today, Mark Tengi and I release an online application to help diagnose whether partisan gerrymandering is evident in a set of election results. The application is intended for the use of judges, clerks, litigants, and others who want a statistically well-founded and easily understood test for partisan asymmetry. The Supreme Court has suggested that partisan asymmetry may form a basis for a manageable standard for partisan gerrymandering, but they have not settled upon a specific standard. I hope to fill that gap.
The website, gerrymander.princeton.edu, implements three tests for partisan gerrymandering as described in an article I published last week in the Stanford Law Review. These proposed standards recently won a prize in Common Cause’s 2016 contest to define a partisan gerrymandering standard. The website is in beta-test, and I welcome your comments. If you detect a problem, email the output PDF if possible.
My three standards have two key features: (1) they implement the principle of partisan asymmetry, as others have also recently done; and (2) they do so without the use of any consideration of maps.
The second point is quite important. Most people who get exercised at the offense of gerrymandering may gravitate toward examination of a district’s convoluted boundaries. Although this is perfectly reasonable, existing precedents and consequences of the Voting Rights Act have conspired to make consideration of boundaries a tough sell with courts – at least for statewide partisan gerrymandering. Let me explain.
The word “gerrymandering” encompasses two kinds of offense.
First, an individual district can be drawn to give an overwhelming advantage to one party or candidate. To paraphrase legal scholar George Berman, this consists of a legislator choosing his/her voters, and not the other way around. The Supreme Court has said that districts should be “compact.” However, they have also allowed that compactness could be geographic…or community-based. This means that a strange shape is permissible. In addition, Section 2 of the Voting Rights Act (as well as the late Section 5, which was killed by the Shelby County v. Holder decision) mandated the creation of districts that empowered specific communities of interest such as blacks or Hispanics. Since the 1970s, district boundaries have gotten more complicated, probably because of this mandate. So an argument that a district’s boundaries are convoluted can be countered with the defense that it was necessary and/or legally permissible.
The second kind of gerrymander is partisan: the construction of an entire statewide scheme, composed of single-district gerrymanders, that gives a net overall advantage to one party. Here, the consideration of a single district’s boundaries is again ambiguous. Why? Because any individual district can be part of a statewide scheme that is neutral overall, or helps one party. For example, if all districts in a state were drawn to be 60-40 for either party, the overall effect would not give either side an advantage. But a party could pack most of its opponents into a tiny number of 80-20 districts, and get more 60-40 districts for itself. A given 60-40 district could potentially look the same in both schemes. So an argument based on the properties of single districts may not be logically watertight.
I therefore suggest that any standard for diagnosing partisan gerrymandering should consider all districts in a state as a whole. Apprehending lots of data at once in a single measure is the raison d’être for the field of statistics.
The online app calculates three statistical quantities:
- The presence of lopsided wins for one party but not the other. This calculation uses a two-sample t-test, perhaps the most widely used statistical test in the world.
- The construction of consistent wins for one party. This can be measured two ways: by the mean-median difference to detect overall skewness, or a chi-squared test to test whether one party’s wins are too consistent to have occurred by chance.
- The number of seats that a party has gained in excess of what would arise naturally, given national population-clustering patterns. This is done by calculating 1,000,000 hypothetical “fantasy delegations” to see what would arise if redistricters were not seeking a systematic statewide advantage.
The first two tests can be done pretty easily; one objective of gerrymander.princeton.edu makes it fairly painless to calculate them. In addition, the website gives you the ability to calculate the third test at the press of a button. As it turns out, a map-less approach to calculating fantasy delegations can be done extremely fast. One million delegations can be simulated in well under a minute.
If you want to consider specific district boundaries, there has been lots of effort in that direction. One example is a random-map-drawing approach taken by Jowei Chen and John Rodden. Another is the first-place winner in this year’s Common Cause contest, by Wendy Tam Cho and Yan Y. Liu. These approaches go into great geographic detail, and are complementary to my proposal. However, they have the disadvantage that a judge would have difficulty applying them without the help of an expert witness. My hope is to place a tool into the hands of a judge that he or she could apply directly – even by jotting it in the margin of a brief.
I hope one or more of these standards will find a receptive audience at the Supreme Court in the near future. No matter whose standard is adopted, it will be a substantial improvement over the current limbo. You can read more about that limbo in my article.