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Census Redistricting Data
Title 13 of the U.S. Code in Public Law 94-171 requires the Census Bureau to distribute specified census data to governors, legislative bodies or other officials responsible for redistricting, by Spring in the year following the census. While most states use this decennial census data for redistricting, 21 states require use of this data in statute.
It is important to note that the decennial census data includes more than the demographic profile of the country’s population. It also includes specific detailed geographic information on nearly every geographic feature imaginable, including streets, water features, political boundaries, election precincts, prisons, and many other land features.
Census Geography
Census data is collected from individuals and households, but it is not reported by address. Instead, the census aggregates information by census block, which in urban areas is typically the size of one city block. Generally speaking, these “blocks” are grouped into “block groups,” which make up larger census tracts which build up into counties and then to entire states.
Past and present questions and controversy surrounding census data abound, and many directly affect the redistricting process such as whether to include a citizenship question and the use of adjusted census data.
Citizenship Question: The U.S. Census Bureau was directed by the Department of Commerce to add a citizenship question on the 2020 decennial census questionnaire, but a federal district court held that the process by which this directive was made violated the federal Administrative Procedure Act. The Supreme Court remanded the federal court decision in 2019. (Dept. Of Commerce v, New York, No. 18-966, 588 U.S. ___) The 2020 census ensued without a citizenship question; however, the President has issued a directive for the Census Bureau to determine citizen population from government administrative records and augment the decennial census results with this data.
It is important to note, that the Census Bureau historically estimates citizen population through periodic surveys each year (American Community Survey, or ACS). That data, however, provides only an estimate that is only considered accurate down to a geographic area that is comparable to a portion of a census tract. The president’s directive, if achieved, is aimed at providing more detailed citizenship data that would be available at the smallest level of geography; a census block, which in many cases, is about the size of a street block.
Adjusted Data: In the past, some states have sought to adjust the decennial census data for various reasons such as to exclude nonresidents or military members stationed within the state. Courts have allowed this in some circumstances but only where the alternative population data has been reliable and accurate when compared to U.S. census data. More recently, states have been attempting to rectify representation issues caused by “prison gerrymandering,” which can cause voters in districts with large prison populations to have relatively more influence at the polls. To combat this, a growing number of states have passed statutes to require the decennial census data to be “adjusted” so that prisoners are not counted at the prison (as is the official practice of the Census Bureau) but at their most recent residential address. Currently, six states currently do this. Read more about prisoner reallocation here.
Differential Privacy: Through the decades, the U.S. Census Bureau has used methods to ensure that the privacy of individuals is maintained when reporting census results. Technological advances in data mining have made it possible to uncover individual personal data by using census databases in conjunction with other public or commercial databases. Differential privacy is a statistical method used to combat this practice, and it is being implemented by the Census Bureau for the first time in the 2020 census. You can read more about this at the Census Bureau’s Differential Privacy information page as well as on Redistrictingonline.
Redistricting Technology
A process that was once accomplished by marking up large paper maps and handwritten voting precinct lists, is now fully automated. The entire redistricting process, while complex, can now be fully executed on a laptop using GIS software. As computing power has increased exponentially over the decades, so has the number of map possibilities and the accuracy with which line-drawers may carry out their goals. This increased preciseness is directly related to increased litigation and more complex legal questions.
At the same time, technology has also opened up a once dark and murky process to the general public. The public may participate in the redistricting through online software that many states host for public input and via readily available software packages that are either free, or relatively low cost.
Redistricting GIS Software: Major software suppliers for redistricting include ESRI and Caliper Corp. ESRI is a popular general GIS software used by geographers and other professionals that includes an add-on for the redistricting function. Caliper Corp is a GIS software concern that designed and developed a separate software package designed specifically for redistricting.
There are several other GIS providers and a host of free online sources as well. In addition, some states build their own custom software to complete the task and others allow the public to develop and submit plans online. District Builder is a popular open-source software redistricting application designed “to give the public transparent, accessible, and easy-to-use on-line mapping tools” according to the Public Mapping Project.
Automated Redistricting Academics have also developed computer algorithms that produce hundreds and sometimes thousands of district map configurations based on a priority list of features. This process has computational complexity. While automated redistricting has not been the preferred option for states and local governments, it is used by academics to study the likelihood that a computer could duplicate a human-drawn map that is thought to be a gerrymander.
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