Redistricting: Drawing the Line

TL;DR

This paper introduces a method to evaluate the fairness of political districting by generating numerous non-partisan redistrictings and analyzing their election outcome distributions, highlighting the representativeness of actual district plans.

Contribution

We develop a Monte Carlo-based approach to assess districting fairness, providing a statistical baseline and critique of existing and proposed district maps.

Findings

2012 and 2016 districtings are highly atypical and unrepresentative.

A bipartisan panel's districting is highly typical and representative.

Generated redistrictings reveal a broad range of possible election outcomes.

Abstract

We develop methods to evaluate whether a political districting accurately represents the will of the people. To explore and showcase our ideas, we concentrate on the congressional districts for the U.S. House of representatives and use the state of North Carolina and its redistrictings since the 2010 census. Using a Monte Carlo algorithm, we randomly generate over 24,000 redistrictings that are non-partisan and adhere to criteria from proposed legislation. Applying historical voting data to these random redistrictings, we find that the number of democratic and republican representatives elected varies drastically depending on how districts are drawn. Some results are more common, and we gain a clear range of expected election outcomes. Using the statistics of our generated redistrictings, we critique the particular congressional districtings used in the 2012 and 2016 NC elections as well as a districting proposed by a bipartisan redistricting commission. We find that the 2012 and 2016 districtings are highly atypical and not representative of the will of the people. On the other hand, our results indicate that a plan produced by a bipartisan panel of retired judges is highly typical and representative. Since our analyses are based on an ensemble of reasonable redistrictings of North Carolina, they provide a baseline for a given election which incorporates the geometry of the state's population distribution.