Understanding and Mitigating the Impacts of Differentially Private Census Data on State Level Redistricting

TL;DR

This paper investigates how differential privacy in the 2020 Census affects redistricting, showing that noise can be understood and mitigated, thus supporting fair district drawing despite privacy measures.

Contribution

It provides a detailed analysis of privacy-induced discrepancies in redistricting data and proposes models to account for and mitigate these effects.

Findings

Thresholding amplifies privacy noise impact.

Simple models effectively capture discrepancies.

Proposed mitigation improves district population balance.

Abstract

Data from the Decennial Census is published only after applying a disclosure avoidance system (DAS). Data users were shaken by the adoption of differential privacy in the 2020 DAS, a radical departure from past methods. The goal of this paper is to better understand how the perturbations from the 2020 DAS combine with sharp legal thresholds to impact redistricting. We consider two redistricting settings in which a data user might be concerned about the impacts of privacy preserving noise: drawing equal population districts and litigating voting rights cases. What discrepancies arise if the user does nothing to account for disclosure avoidance? How can the discrepancies be understood and accounted for? We study these questions by comparing the official 2010 Redistricting Data to the 2010 Demonstration Data--created using the 2020 DAS--in an analysis of millions of algorithmically generated state legislative redistricting plans. We find that thresholding can amplify the impact of the noise from disclosure avoidance. Large discrepancies do occur, but in ways that are well-captured by simple models and appear to be possible to account for. We demonstrate the utility of these models by proposing an approach to mitigate discrepancies when balancing district populations. At least for state legislatures, Alabama's claim that differential privacy "inhibits a State's right to draw fair lines" lacks support.