Partisan Gerrymandering and Political Geography: A Simulation-Based Analysis on the Effectiveness of the Efficiency Gap and Mean–Median as Measures of Partisan Gerrymandering

Abstract

Gerrymandering has become much more frequent in the United States in the past 20 years. Researchers have developed measures, such as the efficiency gap, to help courts adjudicate whether a redistricting plan is an outlier compared with other nonpartisan plans. This study evaluates two of these measures: the efficiency gap and the mean–median difference. Using a simulation approach, I evaluate whether current implementations of these measures accurately determine whether a district is an outlier relative to simulated maps generated without any explicit partisan consideration. The results show that the threshold currently used for the efficiency gap measure is arbitrary and inefficient and is sensitive to the geographic distribution of voters from each party. For the mean–median difference, the results raise serious concerns about the basic assumption behind the measure’s design.

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