Can Gerrymanders Be Measured? An Examination of Wisconsin's State Assembly

Abstract

In July, 2015 a group of Wisconsin Democrats filed a federal lawsuit claiming that the state’s Assembly map adopted after the 2010 census discriminates against Democrats by systematically underweighting their votes relative to Republicans. This claim of a constitutional violation rooted in the 14th Amendment offers judges a rationale to constrain partisan gerrymandering – provided the plaintiffs can produce objective and compelling evidence that a gerrymander has been created. In this paper we evaluate the Wisconsin Assembly map using a pair of methods proposed to detect and measure gerrymanders, the “efficiency gap” measure employed by the plaintiffs in Wisconsin and the median-mean comparison proposed by the authors. Additionally, we use an innovative new procedure to produce a comparison set of 10,000 neutral maps drawn by computer. The results show that the Assembly map in Wisconsin is clearly a fairly egregious gerrymander using the median-mean comparison but not via the efficiency gap calculation. We examine the measurement qualities of the efficiency gap to explain its shortcomings, especially in Wisconsin, and review the remaining evidence to conclude that Wisconsin’s Assembly map is the substantial pro-Republican gerrymander that the plaintiffs claim it to be despite dubious results using the efficiency-gap calculation.