Glossary of Redistricting Terms - Redistricting Learning Portal

Aggregation	Summing numeric values to a larger geographic feature from a smaller feature.
Absolute Deviation	The difference between the district’s population and ideal population.
Balancing Field	The field that contains data that is being equalized across districts in a plan. In most plans this field is Population.
Base Layer	Building blocks for a plan, usually the smallest features in area, Census Blocks.
Census	A 100% enumeration of the population; the federal census is mandated by the U.S. Constitution in Article 1, section 2.
Census Block	The most granular and smallest geography for decennial census tabulations. In urban areas, they are similar to a street block. Census blocks are generally bounded by physical features such as roads, rivers, or power lines but can sometimes be bounded by nonphysical features such as city, county or precinct boundaries
Census Block Group	Statistical divisions of census tracts, and generally contain between 600 and 3,000 people. Block groups tend to follow neighborhoods. They are used to present data and control block numbering. A block group consists of clusters of blocks within the same census tract that have the same first digit of their four-digit census block number. Most block groups were delineated by local participants in the Census Bureau’s Participant Statistical Areas Program.
Census Bureau	The U.S. Bureau of the Census, which part of the Department of Commerce, conducts the decennial Census of Population and Housing as well as numerous ongoing projects for the federal government.
Census Geography	The geographic units for which census information is tabulated and reported with several hierarchies; the most basic is census block to census block group to census tract to county to state.
Census Tract	Census tracts are small, relatively permanent geographic entities within counties (or the statistical equivalents of counties) delineated by a committee of local data users. Generally, census tracts have between 2,500 and 8,000 residents and boundaries that follow visible features. When first established, census tracts were to be as homogeneous as possible with respect to population characteristics, economic status, and living conditions. Tracts were first defined in 1970 and the Census Bureau maintains them as consistently as possible across the decades.
Citizen Voting Age Population (CVAP)	Number of persons who are citizens and at least 18 years of age.
Communities of Interest	Geographical areas, such as neighborhoods of a city or regions of a state, where the residents have common demographic and/or political interests that do not necessarily coincide with the boundaries of a political subdivision, such as a city or county. Examples of communities of interest are groups who are committed to preserving a local park, creating a new subway line in a city or achieving increased funding for a community college.
Compactness	Having the minimum distance between all the parts of a constituency (a circle, square or a hexagon are examples of very compact district). Various methods have been developed to measure compactness.
Contiguity	All parts of a district being connected geographically at some point with the rest of the district. Limits on contiguity by point or by water vary by state.
Deviation	The measure of how much a district or plan varies from the ideal population, however defined, per district. Deviation can be expressed as an absolute number or as a percentage.
Differential Privacy	(Also known as Disclosure Avoidance) The Census Bureau’s method that adds “statistical noise” to the population counts in a way that protects each respondent’s identity. Differential privacy uses statistical noise to slightly alter data so that the link between the data and a specific person or business can’t be certain. With differential privacy, the Census Bureau precisely controls the amount of statistical noise added using sophisticated mathematical formulas that allow them to assure that enough noise is added to protect privacy but not so much as to damage the statistical validity of their publications. The formulas allow the bureau to balance between two opposing extremes: total accuracy and total privacy. In practice, this means that areas that have a large number of people will have highly accurate statistics, but areas or subpopulations that have just a few people will have proportionately more noise and therefore less accuracy. In this way, the statistical noise prevents someone from learning anything meaningful about any particular individual. You can read more about differential privacy in this article.
Disaggregation	The process of apportioning data for a larger level of geography to a lower level.
District	The boundaries that define the constituency from which a public official is elected.
Ensembles	Groups of randomized redistricting plans created using a proprietary implementation of the ReCom method of the Markov chain Monte Carlo algorithm. These plans can be compared to any proposed plan to determine how random it is.
Equivalency File	A file that contains the unique identifier for the base layer, usually the Block ANSI for Census Blocks, and the corresponding district identifier. Equivalency files can be used to recreate district geography using any GIS.
GIS	Geographic Information System. Computer software used for creating or revising plans and analyzing geographically oriented data.
Hierarchy	Geographic entities in a superior/subordinate structure. This structure is derived from the legal, administrative, or areal relationships of the entities. Hierarchies contain a series of nesting relationships. Smaller orders in the hierarchy are completely contained by larger orders. Common hierarchies are Census Block-Block Group-Tract-County, Census Block-Voting District-County, Census Block-Voting District-County Subdivision-County.
Ideal Population	The total population or alternative for the redistricting divided by the number of seats in a plan.
Layer	A group of features of the same type, such as states, highways, or landmarks that is stored in geographic files on your computer.
Library	A collection of plans in the Plan Manager.
Majority-Minority District	Term used by courts for seats where a group or a single racial or language minority constitutes a majority of the population. (These are also referred to as “effective districts.”)
Map	A representation of an area of land or sea showing physical features (i.e. cities, roads, etc.) shown as layers.
Maptitude Online Redistricting (MORe)	Caliper Corporation’s online application for creating interactive redistricting maps.
Metes and Bounds	A detailed and very specialized description of district boundaries using specific geographic features and street directions as are usually found in describing real property for legal purposes.
Multi-Member District	A district that elects more than one member to a political jurisdiction.
Nesting	Redistricting method of creating two or more state lower legislative chamber districts that are completely contained within the boundaries of a state upper legislative chamber district.
Overall Range	The difference in population between the largest and smallest districts in a districting plan in either absolute (persons) or relative (percentage) terms.
Plan	A set of boundaries for all districts of a representational body, a.k.a. map.
Plan Manager	The utility for creating and managing plans, libraries, templates, plan sets and reports in Maptitude for Redistricting.
Plan Sets	Two or more related plans that you want to work on simultaneously.
Plurality	A percentage of population that is higher than all other race/ethnicities.
Public Law 94-171	Federal law enacted in 1975 requiring the U. S. Bureau of the Census to provide the states with data for use in redistricting as well as mandating the program where the states define the geography for collecting data.
Redistricting	The redrawing or revision of boundaries for representational districts.
Redistricting Toolbox	The toolbox that contains all of the tools to adjust districts in a redistricting plan.
Selection Layer	The features that are being moved from the Source district to the Target district.
Standard Deviation	A statistical formula measuring variance from the average for the entire set of data.
Source	The district from which selected features are being removed in a redistricting plan.
Tabulation	The totaling and reporting of the census data from individual responses for all levels of census geography.
Target	The district which selected features are being added to in a redistricting plan.
Template	A collection of all the setup information for a plan but none of the district assignments.
Theme	The use of distinctive colors, symbols, and fill styles to portray data such as population, race, or ethnicity. The different styles highlight the similarities and differences among map features.
TIGER	Topologically Integrated Geographic Encoding and Referencing. The system and digital database developed at the U.S. Census Bureau to support computer maps used by the census.
Travel Contiguity	The ability to travel across a district without leaving a district. Districts with higher travel contiguity are preferred.
Voting Age Population (VAP)	The number of persons 18 years of age and over.
VTD (Voting District)	A census term for a geographic area, such as an election precinct, where election information and data are collected; boundaries are provided to the Census Bureau by the states. Since boundaries must coincide with census blocks, VTD boundaries may not be the same as the election precinct and may include more than one precinct.