Census Research

It is important to understand the geography of the Census because different items of information are available for different units of geography. Simply knowing the "normal" geography of an area isn't always sufficient since the Census Bureau defines some geographical areas using different boundaries and different terminology. The Census Bureau does attempt to make its geographical areas conform to what an area is really like, however, and they use local input whenever possible.

An important geographic maxim: 
The smaller the geographical area, the fewer items of data are available

Census Tracts are the smallest areas for which the important income, education and other social data (derived from surveys) items are available. Blocks are the smallest area for which the Census reports data, and here the items available are largely the basic ones from the 100% counts: age, sex, and race.

An important reason for this is confidentiality. A fundamental Census principle is that they will not display any data that can be used to identify a particular person, household or other basic counting unit. There might very well be so few people in an income class, for example, in a certain block, that reporting the numbers would actually give information on a particular household.

Census.gov Information

Other Sources on Census Geography Topics

Here are the major Census geographical terms and their definitions.

• Census Block (or, simply, Block). The smallest unit for which data is reported; all larger areas are aggregations of Blocks. A block contains 1 or more housing units for which data is collected.
  • In urban areas, blocks conform to what is usually meant by a city block, i.e., a small area bounded on all sides by streets. Other types of boundary lines may be used when necessary, such as railroad tracks, streams, city and county lines, etc.
  • Blocks cover the entire nation for Census purposes. In rural or remote areas, then, Blocks may be large (many square miles) in area and irregular in shape. For Census data reporting purposes, one kind of block is the same as another.
  • Blocks vary widely in population, and there are 7 million Blocks nationwide. Each one is given a four-digit identification number within its Census Tract (see below).
• Block Group. A cluster of Census Blocks, all of which share the same first digit of their four-digit identification number.
  • Ideally contain about 1500 people, with ranges generally between 600 and 3000.
  • Never split between counties; since they are always located wholly in a single county, block data can be neatly aggregated for the county.
  • Block Groups cover the entire nation.
• Census Tract. Tracts are larger than Block Groups but still relatively small areas, and are, ideally, fairly stable county subdivisions; they are usually delineated with local input. Tracts are particularly important geographic units, as they are the smallest for which detailed sample data is reported. 
  • Usually have populations between 2500 and 8000, and as a consequence they vary considerably in size.
  • The 2000 Census is the first one for which the entire country has been divided into Census Tracts; previously, Tract data was not available, as such, for mostly rural areas.
  • Tracts are never split between counties; since they are always located wholly in a single county, tract data can be neatly aggregated for the county. However, Tracts might be spilt between Places, however (see below).
• Place. This is the generic name the Census Bureau gives to cities, towns, boroughs, and villages. They can be incorporated or not (see below). Keep in mind that city boundaries might not always match reality on the ground. For example, unincorporated areas outside of a city might be the location of economic activity that's normally associated with the city.
  • Census Designated Place (CDP). Census-determined entity, a settled concentration of population that is identifiable by name but is not legally incorporated. This is an important definition in highly rural states or areas.
• County Subdivisions.
  • Minor Civil Divisions. In the Northeast, these are formal government structures at the sub-county level.
  • Census County Divisions. The Census Bureau has extended the concept of Minor Civil Divisions to other parts of the country, creating these sub-county units for statistical purposes. They have stable boundaries and recognizable names.
• Metropolitan Area (MA).Areas defined by the Office of Management and Budget for the purposes of collecting and reporting statistical data. An Area will include a large city or cities, typically, with their surrounding suburbs. The key point is that the suburbs are closely integrated, socially and economically, with the central city. Terminology has varied over time on this topic, with the Areas referred to as Standard Metropolitan Areas (SMA) from 1949-1959, Standard Metropolitan Statistical Areas (SMSA) from 1959-1982, Metropolitan Statistical Areas (MSA) from 1983-1989, and, currently, simply Metropolitan Areas.

Here are some more terms dealing with urban areas:

• Urbanized areas.  Areas where there is population-based impact on the landscape, largely based on population density but also including considerations such as impervious surface cover.
  • Urban Areas have 50,000 or more inhabitants
  • Urban Clusters are smaller, more rural areas that contain from 2500 to 50,000 people.
• Core-based Statistical Areas (CBSA). Determined by the Office of Management and Budget; county-based.
  • All federal statistical agencies provide data for CBSAs; this data might be referred to as Metropolitan/Micropolitan data.
  • CBSAs aim to identify areas with a high level of social and economic integration.
  • Metropolitan counties have Urbanized Areas within their boundaries
  • Micropolitan counties have an Urban Cluster within them, but not an Urbanized Area
  • Groups of counties with social and economic integration can have their data aggregated into a CBSA.
• Zip Code Tabulation Area (ZCTA). Areas that approximate Postal Service 3- and 5-digit Zip Code areas, with some adjustments so as not to truncate Census Blocks. 
  • Generally speaking, demographic Census data is not available by Zip Code, but by ZCTA; Economic Census data can be available by Zip Code.
  • For demographic purposes, the most frequently-occurring postal Zip Code in a Census Block is assigned to the ZCTA.
  • This geographical category was new with the 2000 Census,
  • ZCTA's cover the whole nation, but exclude large bodies of water and large upopulated areas. 
• States/Counties. These geographical areas are [thankfully] not modified by the Census Bureau, but retain their normal definitions and boundaries.

While not as challenging as comparability problems arising from differences in questions asked, geographical changes can be very important nonetheless. 

Metropolitan areas. As you can imagine, in the ten years between Censuses, new cities will attain Metropolitan Area status (although the changing definitions applied to urban areas account for some of the differences, too. And the boundaries of MAs will shift considerably over time as well, making comparability of data for some applications difficult.

Census tracts. Tracts are also variable over time, since shifts of population concentrations may necessitate new boundaries. 

Even such supposedly stable geographic entities as counties can be changed over time.