An Introduction to Partisan Gerrymandering Metrics

It is sometimes felt that gerrymandering is such an obvious form of chicanery that any political body sincerely desiring to achieve a completely equitable redistricting should have no trouble in carrying out such a purpose . . . But whenever the drawing up of the boundaries is left even slightly to the discretion of an interested body, considerable latitude is left for the exercise of the art. [1]

The most obvious defense against partisan gerrymandering1 is for disinterested parties to conduct redistricting processes in the open, using open source analytical tools, with the purpose of creating districts that will satisfy existing regulations and meet the intentions of both U.S. and state constitutions. Such a process would not eliminate the need for metrics capable of identifying partisan bias, but it would change how we use them. Rather than uncovering the unjust handiwork of partisan groups, the metrics would be used to guide the selection of the best redistricting products created by well-intentioned teams; a significantly easier threshold to meet (i.e., picking a good-enough redistricting map instead of trying to reveal the unknown intentions of a partisan group). Until the states institute such a process, we are left to rely on the courts to judge.

Analysts continue to develop and evaluate metrics that could be used to identify the existence of partisan gerrymandering. [2] However, one of the greatest difficulties they face is deciding how best to respond to the Supreme Court’s desire for a simple measure to detect gerrymandered districts given the sophisticated tools being used to create them. The primary partisan gerrymandering metric used until recently has been symmetry; however, it has a number of weaknesses that keep it from serving as a legal standard. Namely, it relies on artificial data and, despite decades of use, has yet to identify a value above which partisan gerrymandering can be said to exist. Although it is fairly new, the efficiency gap metric has a significant body of research behind it. Moreover, it measures a quantity that is very similar to symmetry, only uses data from the election of interest, and it has a proposed standard. It is possible that the efficiency gap metric could fill this legal void. The challenge to Wisconsin’s redistricting plan used the efficiency gap metric as the foundation of its argument for the existence of gerrymandering. Oral arguments were held in early October 2017, but it is unclear how the Court will rule.

Partisan gerrymandering metrics like those introduced here are the only means to ensure our democracy is based on “one person, one vote.” Therefore, it is important for citizens to attempt to understand them.