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Glossary of Terms
American Community Survey (ACS): An ongoing census survey sent to a sample of three million housing units annually. The ACS collects detailed demographic and socioeconomic population and housing characteristics, similar to the information collected on the former long form census questionnaire. The data is collected continuously rather than once a decade, so the ACS provides more current data. Single-year estimates are available annually for geographic areas with a population of 65,000 or more. Three-year estimates are available annually for geographic areas with a population of 20,000 or more. In December 2010, the first five-year estimates at the census tract and block group level will be available.
Anglo: Those persons who identified their race on the census form as White only and not Hispanic.
Assignment unit: Any unit of geography that may be used as a building block to draw a redistricting plan. Assignment units available in RedAppl are counties, census tracts, census block groups, census blocks, and VTDs (voting tabulation districts).
Black: Those persons who identified their race on the census form as Black, African American, or Negro only or Black and any other race. Black persons can be either Hispanic or non-Hispanic.
Black + Hispanic: A combined population category that includes all persons who identified their race as Black and all persons who identified themselves as Hispanic. The total is adjusted so that those who indicated they were both Black and Hispanic are not counted twice. The category is frequently examined for redistricting purposes in areas in which Black and Hispanic voters may form political coalitions or vote together as a bloc.
Census block: The smallest unit of census geography for which population data are counted and reported. Census blocks are delineated by the Census Bureau and are generally bounded by physical features such as roads, creeks, or shorelines, but also may be bounded by nonvisible features such as city, county, school district, or voting precinct boundaries.
Census block group: A subdivision of a census tract composed of a group of contiguous census blocks.
Census day: April 1, 2010. The date for which census data is collected.
Census Designated Place (CDP): A densely settled, unincorporated area locally identified by a name, such as an unincorporated town, for which the Census Bureau reports population. The boundaries of a census designated place are established by the Census Bureau in cooperation with state and local government officials.
Census tract: A unit of census geography delineated by local committees in accordance with census bureau guidelines for the purpose of collecting and presenting decennial census data. Census tracts are made up of block groups. Their boundaries generally follow visible features, though in some circumstances their boundaries may follow governmental unit boundaries or other nonvisible features. In general, census tracts contain between 1,000 and 8,000 inhabitants, with an average population of 4,000.
Community of interest: A term sometimes used to describe a grouping of people in a geographical area, such as a specific region or neighborhood, who have common political, social, or economic interests.
Compactness: Three basic types of compactness measures are used to analyze the area, perimeter, and population of a district. More than 36 different variations on these compactness measures exist. No single measure of compactness is recognized as the "best" measure. The measures presented in RedAppl and on reports have a range from zero to one, with one indicating perfect compactness for the particular scale. The lower the score, the less compact the district.
Types of Compactness Measures:
- Area Dispersion: This measure examines the relative degree to which a district's area is compact when compared with the area of a similar compact figure. It is the ratio of the area of the district to the area of the smallest convex polygon that can enclose the district (imagine a rubber band stretched around the district). A district in the shape of a square would receive a perfect score of one using this measure. This measure penalizes a district that has long "fingers" or extensions, making it less compact because it requires a larger convex polygon to enclose the entire district, yet much of that polygon is empty.
- Perimeter: This measure compares the relative length of the perimeter of a district to its area. It is the ratio of the area of the district to the area of a circle with the same perimeter as the district. A perimeter-to-area measure penalizes a district's compactness score whenever the boundaries are uneven or irregular: the more the boundary zigzags (for example, a river), the less compact the district using this measure.
- Population: This is a population-based compactness measure computed as the ratio of the population of the district to the population of all census blocks contained in the smallest convex polygon enclosing the district. The greater the population within the convex polygon that is not within the district, the less compact the district under this measure.
Contiguity: Adjacency. For redistricting purposes, a district is considered to be contiguous if all parts of the district touch one another at more than a point, so that the entire district is within a continuous boundary. Legal standards governing redistricting for various governmental bodies often require all of the territory in each district to be contiguous.
County election precincts: Also called voting precincts. Geographic units established by county commissioners courts for the purpose of election administration. The voters in an election precinct usually vote at a single polling place, so the votes cast in the precinct may be counted separately from other precincts.
