From the 2010 Census Geographic Terms and Concepts:
Tracts Census Tracts are small, relatively permanent statistical subdivisions of a county or equivalent entity that are updated by local participants prior to each decennial census as part of the Census Bureau's Participant Statistical Areas Program. The Census Bureau delineates census tracts in situations where no local participant existed or where state, local, or tribal governments declined to participate. The primary purpose of census tracts is to provide a stable set of geographic units for the presentation of statistical data.
Census tracts generally have a population size between 1,200 and 8,000 people, with an optimum size of 4,000 people. A census tract usually covers a contiguous area; however, the spatial size of census tracts varies widely depending on the density of settlement. Census tract boundaries are delineated with the intention of being maintained over a long time so that statistical comparisons can be made from census to census. Census tracts occasionally are split due to population growth or merged as a result of substantial population decline.
Census tract boundaries generally follow visible and identifiable features. They may follow nonvisible legal boundaries, such as minor civil division (MCD) or incorporated place boundaries in some states and situations, to allow for census-tract-to-governmental-unit relationships where the governmental boundaries tend to remain unchanged between censuses. State and county boundaries always are census tract boundaries in the standard census geographic hierarchy. Tribal census tracts are a unique geographic entity defined within federally recognized American Indian reservations and off-reservation trust lands and can cross state and county boundaries. Tribal census tracts may be completely different from the census tracts and block groups defined by state and county.
Block groups Block Groups (BGs) are statistical divisions of census tracts, are generally defined to contain between 600 and 3,000 people, and 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. For example, blocks 3001, 3002, 3003, . . ., 3999 in census tract 1210.02 belong to BG 3 in that census tract. Most BGs were delineated by local participants in the Census Bureau's Participant Statistical Areas Program. The Census Bureau delineated BGs only where a local or tribal government declined to participate, and a regional organization or State Data Center was not available to participate.
2010 criteria and Participant Statistical Areas Program (PSAP)
Two important principles in the tract delineation are:
Historic continuity: A century of census tract use has shown that continuity and comparability in tracts and their boundaries over time are of considerable importance to data users. The federal register reads: "The Census Bureau discourages changes to tract boundaries
(that is, ‘‘retracting’’), except in specified circumstances, which the Census Bureau will review on a case-bycase basis."
Optimal population size: If the population in tract is too small, the sample size in the American Community Survey gets too small for reliable outcomes. In 2010 the optimal population size was set at 4,000 with a minimum of 1,200 and a maximum of 8,000.
2010 Participant Statistical Areas Program
In the years leading up to the 2010 Census all interested governments, organizations and individuals are strongly encouraged to
participate in the 2010 PSAP review and update so that the resulting 2010 census tracts, block groups, CDPs and CCDs meet the communities’
needs. This document lays out Information for the 2010 PSAP Participants.
2020 Participant Statistical Areas Program and tract and blockgroup delineation
Geographic Partnership Programs conducted in support of the 2020 Census (including PSAP) will be similar in
design to the programs last decade.
Assumption for the design of the 2020 PSAP:
2020 Census PSAP will include all applicable tribal statistical entities.
2020 Census PSAP will consist of an internal review (creating proposed 2020 Census
statistical geographies) as well as external review and verification phases.
2020 PSAP participants will have the option of creating their PSAP delineations using
either the proposed 2020 Census statistical geographies (created during the internal
review) or modifying existing 2010 Census statistical geographies.
With the exception of some tribal governments, 2020 Census PSAP participants will be
required to use the GUPS tool.
All American Indian and Alaska Native areas are solicited for interest in defining tribal
statistical areas under PSAP.
The Census Bureau will engage PSAP participants from as many tribal, state, and local
governments/planning agencies as possible in order to maximize local input into the 2020
Census statistical areas.
Federal Register notices to establish criteria (preliminary asking for comments, followed by final criteria)