Can Gerrymanders Be Detected? An Examination of Wisconsin’s State Assembly

Abstract

In October 2017, the Supreme Court heard an appeal of a November 2016 ruling striking down Wisconsin’s State Assembly districts as a Republican gerrymander that illegally dilutes the weight of Democratic votes. We take the opportunity to revisit this litigation to evaluate three proposed methods of detecting gerrymanders: the “efficiency gap,” a count of Assembly districts carried by statewide candidates, and the difference between the district-level partisan median and mean. The first two measures figure either centrally or peripherally in the plaintiffs’ case in Wisconsin, while the third is the approach we favor. We expand on the analysis offered at trial by evaluating how these measures fare across a variety of elections in Wisconsin and with the aid of 10,000 alternative Assembly maps drawn by computer. The alternative maps provide the appropriate baseline with which to gauge the level of vote dilution in Wisconsin and distinguish between the effect of residential geography and the Legislature’s actions. The results show that Wisconsin’s Assembly map is a substantial gerrymander according the median–mean comparison across all elections, while the two tests relied upon by the plaintiffs provide mixed results. We examine the measurement qualities of each test and show that the efficiency gap and districts-carried count both capture elements beyond partisan bias. We find no similar ambiguity with the median–mean comparison and conclude that the plaintiffs’ claim that Wisconsin’s Assembly map systematically dilutes the weight of Democratic votes is correct.

Evaluating Legislative Districts Using Measures of Partisan Bias and Simulations

“…A related question, especially for the EG, is whether partisan bias should be measured using the outcomes of the legislative elections themselves or with respect to another statewide office such as governor. The EG is sensitive to small changes in vote in highly competitive districts because it relies on them to distinguish seats won, but the meanmedian is more robust to the choice of election used (Krasno et al, 2019). One benefit of using an "exogenous" office is that it avoids the problem of uncontested races that we just highlighted.…”

“…It thus provides a common baseline for evaluating voter preferences across the state. But Krasno et al (2019) have questioned how researchers are to identify the proper statewide elections to use as a baseline for measuring bias. The choice of which race to use might merely be a nuisance because it seems arbitrary and each race generates different values for the EG.…”

“…This would allow for observation of results with different degrees of swing in the vote. The EG is sensitive to small changes in vote in highly competitive districts because it relies on them to distinguish seats won (Campisi et al, 2019), but the mean-median is more robust to the choice of election used (Krasno et al 2019).…”

“…This view also affects how some researchers conceptualize the measures of partisan bias we reviewed. As a result, some researchers are now applying measures such as the mean-median gap and EG measures to simulated maps (see Krasno et al, 2019). This additional step allows scholars to determine the degree to which observed electoral conditions reflect a state's "natural" tendency to favor one party due to residential segregation to make inferences concerning what is an extreme result across the states.…”

The Impact of Partisan Gerrymandering on Political Parties

“…2018; Krasno et al. 2019; Wang 2016), and partisan asymmetry (Gelman and King 1994a). We also complement the dataset with a variety of longitudinal indicators of party health, capturing activities performed by both elites and voters.…”