Redistricting Terms

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Apportionment
The process of assigning seats in a legislative body among established districts.

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Census
Enumeration or count of the population as mandated by the United States Constitution.
Census block
The smallest unit of census geography for which population data are reported. Census blocks are designated by the Census Bureau and are generally bounded by physical features such as roads, creeks, or railroads, though in some cases they may be bounded by non-visible features such as city limits.
Census block group
A cluster of census blocks within a census tract designated by the Census Bureau as a subdivision of that census tract.
Census Day
April 1, 2010:  the date the 2010 census data was collected.
Census tract
A unit of census geography defined by the Census Bureau for the purpose of 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 non-visible features. In general, census tracts must contain between 1,500 and 8,000 inhabitants.
Community of interest
Geographical areas, such as neighborhoods of a city or regions of a state, where the residents have common political interests that do not necessarily coincide with the boundaries of a political subdivision, such as a city or county.
Compactness
The degree to which the territory assigned to a district is close together. There are several mathematical ways to measure the elements of compactness.
Contiguity
All parts of a district connected at some point with the rest of the district.
Cracking
A term used when the electoral strength of a particular group is divided by a redistricting plan.

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Deviation
The amount by which a district's population differs from the ideal district population for the particular plan type.
Differential undercount
The difference between the undercount rate for one group of persons compared to another group.  See "Undercount."
Digital Map Layer
A set of polygons representing geographic units. For redistricting, the primary map layers used include the following:
  • Minor Civil Divisions (MCD): Includes cities, towns, and villages.
  • Voting Tabulation Districts (VTD): Wards inside MCDs, election districts in rural areas.
  • Census Blocks (CNS): The smallest unit of census geography, normally bounded on all sides by visible features such as city or county limits, property lines, and imaginary extensions of roads.
District
The boundaries that define the constituency of an elected official.

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Elections Data
Actual votes for each candidate in primaries and general elections.
Equal Protection Clause
See "Fourteenth Amendment to the U.S. Constitution."

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Fourteenth Amendment to the U.S. Constitution
The U.S. Constitution provision that includes the Equal Protection Clause, which prohibits the states from denying persons equal protection of the law. The Equal Protection Clause is the primary basis of the one-person, one-vote principle.
Fifteenth Amendment to the U.S. Constitution
The U.S. Constitution provision that the right to vote may not be denied or abridged on account of race.
Fragmentation
The division of a geographically concentrated group, such as a racial or political group, among different districts for the purpose of minimizing the group's voting strength.

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Geographic Information System (GIS)
A graphics-based computer system that relates geographic features (such as census tracts, roads, or counties) to data about those features (such as population, race, or income).
Gerrymander
A district or set of districts with unusual boundaries that is drawn in that way to favor one or more interest groups over others.
 

Partisan Gerrymandering: The deliberate drawing of district boundaries to secure an advantage for one political party.

Racial Gerrymandering: The deliberate drawing of district boundaries to secure an advantage for one race.

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Homogenous District
A voting district with at least 90 percent minority or white population.

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Ideal District Population
A population measure equal to the total state population divided by the total number of districts.

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Majority-Minority Districts
Term used by the courts for seats where an ethnic minority constitutes a majority of the population.
Metes & Bounds
A detailed description of district boundaries using specific geographic features.
Method of equal proportions
The mathematical formula used as provided by federal statute to reapportion congressional seats among the states after each decennial census.
Minority vote dilution
The creation of districts that either (1) divide members of a racial or ethnic minority group among several districts, artificially reducing the group's opportunity to influence elections (see "Cracking" and  "Fragmentation") or (2) place extraordinarily high percentages of members of a racial or ethnic minority group in one or more districts, so that minority voting strength is artificially limited to those districts and is minimized in neighboring districts (see "Packing").

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Natural boundaries
District boundaries that are natural geographic features, such as bodies of water.

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One person, one vote
Constitutional standard established by the U.S. Supreme Court that all legislative districts should be approximately equal in population. The courts derive the one-person, one-vote standard primarily from the Equal Protection Clause of the Fourteenth Amendment to the U.S. Constitution. For congressional districts, the one person, one vote requirement also derives from Section 2, Article I, and from Section 2 of the 14th Amendment to the U.S. Constitution.
Overall Range or Overall Deviation
For a redistricting plan, the difference in population between the smallest and largest district, normally expressed as a percentage.

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P.L. (Public Law) 94-171
The federal statute that requires the Census Bureau to provide, by April 1 of each year following a decennial census, the population and race data necessary for redistricting.
Packing
Consolidate a group of voters, such as a racial or political group, in a small number of districts thus tending to result in the election of the group's candidate of choice in any election in that district and the dilution of the group's voting strength in neighboring districts.
Phase I and Phase II
Programs run by the Census Bureau to collect boundary information from state and local governments. Phase I allows states to suggest boundaries for census blocks. Phase II lets state's group blocks into precincts so the official census data will contain precinct population totals.
Population estimates
An approximation of the population of a geographic unit at a point in the past or present for which an actual population count is not available.
Population projections
An approximation of the population of a geographic unit at a point in the future based on specific assumptions regarding future demographic trends in the geographic unit.

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Racially polarized voting
The term used to describe circumstances in which the voting preferences of a racial or ethnic group consistently vary from those of other racial or ethnic groups, particularly when the different voting preferences are based on the race of the candidate. Also referred to as "racial bloc voting."
Reapportionment
Reallocation of a fixed number of seats in a governmental body among established political units. Following each decennial census, the seats in the U.S. House of Representatives are reapportioned among the states. The result is that each state is assigned its number of congressional seats for the next decade. Reapportionment does not result in the establishment of new district boundaries. The term is sometimes used imprecisely to mean "redistricting."
Redistricting
Drawing and redefining political district boundaries.
Retrogression
The term used to describe a reduction in the voting strength of a racial or ethnic group resulting from a redistricting plan or other change in election procedures. Retrogression is the primary test used for evaluating a change in election procedures under Section 5 of the Voting Rights Act.

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Standard Deviation
A statistical formula measuring variance from a norm.
Statistical sampling
The statistical method by which characteristics of a small group are measured and applied to the population as a whole.
Single-Member District
District that elects only one representative.

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Tabulation
The totaling and reporting of the census data.
Topologically Integrated Geographic Encoding and Referencing System (TIGER)
The TIGER/Line files are a digital database of geographic features, such as roads, railroads, rivers, lakes, political boundaries, and census statistical boundaries covering the entire United States. The data base contains information about these features such as their location in latitude and longitude, the name, the type of feature, address ranges for most streets, the geographic relationship to other features, and other related information. TIGER was developed at the Census Bureau to support the mapping and related geographic activities required by the decennial census and sample survey programs.
Total range of deviation
The range over which the populations of all districts in a redistricting plan deviate from the ideal district population.
Traditional districting principles
A term often used to refer to criteria, such as compactness and contiguity, that have historically been considered in drawing election districts.

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Undercount
The estimated number of people who are not counted by the census.

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Voting age population
The number of people over the age of 18.
Voting Rights Act
The federal law prohibiting discrimination in voting practices on the basis of race or language group, codified as 42 U.S.C. Section 1973 et seq. The official title of the Act is the Voting Rights Act of 1965. Section 2 is germane for redistricting in Massachusetts.
Section 2: Prohibits the adoption of voting standards or practices that abridge the right to vote on the basis of race or language group. This section applies to all states and other governmental units and may be used to challenge a redistricting plan that discriminates against a racial or language minority group.
Voting tabulation district (VTD)
The generic US Census name for geographic entities, such as precincts, wards, and election districts, established by state governments for the purpose of conducting elections.