Knowing how many people live in a given area and their characteristics is critical for establishing a baseline understanding of who might be affected by a given project or policy and their current health risks and assets. The U.S. Census Bureau collects various forms of data useful for describing the demographic characteristics and economic status of affected populations. Besides information from the nationwide census that is conducted every ten years, the Census Bureau also provides data from a wide range of community and economic surveys.
- Census tracts: A small, relatively permanent statistical subdivision of a county delineated by a local committee of census data users for the purpose of presenting data. They are designed to be relatively homogeneous units with respect to population characteristics, economic status, and living conditions at the time of establishment, census tracts average about 4,000 inhabitants. They always nest withn counties, but they may be split by any sub-county geographic entity.
- Census block: A subdivision of a census tract, a block is the smallest geographic unit for which the Census Bureau tabulates 100-percent data. Many blocks correspond to individual city blocks bounded by streets, but blocks -- especially in rural areas - may include many square miles and may have some boundaries that are not streets.
- Zip code tabulation area (ZCTA):A geographic area that approximates the delivery area for a five-digit or a three-digit ZIP Code. While ZCTAs are useful for organizing and presenting Census data, they do not precisely depict the area within which mail deliveries associated with that ZIP Code occur.
Special Census tools and data sets
Community Economic Development HotReport http://lehd.did.census.gov/led/datatools/hotreport.html
The Community Economic Development HotReport combines Census and other publicly available data sources to provide “one-stop” access for information on economic and social indicators for individual counties, presented in tables, graphs, and maps. Data include:
Quarterly Workforce Indicators (QWI) http://lehd.did.census.gov/led/datatools/qwi-online.html
QWI combines county- and state-level employment data from various sources and allows users to examine by industry, year, gender and age. The eight QWI indicators include:
Small Area Income & Poverty Estimates (SAIPE) http://www.census.gov/hhes/www/saipe/index.html
With support from other federal agencies the Census Bureau provides model-based estimates of income and poverty statistics for counties and school districts. Data are released annually with about a 2-year delay (i.e. 2006 data will be released in late 2008). SAIPE data include:
American Community Survey (ACS) http://www.census.gov/acs/www/index.html
The American Community Survey, which is sent to a small percentage of the population on a rotating, annual basis, lets communities see how they are changing - filling in the gaps between each 10-year census. The ACS covers the same demographic, economic and housing topics as the Census, but it is based on sampling (not enumeration) and currently focuses mostly on larger geographic The 2006 ACS data (the most recent ACS data available through late 2008) covers geographic areas with populations of 65,000 and greater. In late 2008, the Census Bureau will release data for all geographic areas with populations of 20,000 or larger. In late 2010, the Census Bureau will release data for areas as small as census tracts and block groups, nationwide. So, for very small towns, 2010 will be the first time ACS data are published. ACS can be accessed through American FactFinder. ACS sampling issues are discussed at http://www.amstat.org/Sections/Srms/Proceedings/y2008/Files/302226.pdf.
Economic Census and Surveys http://www.census.gov/econ/index.html
The Economic Census profiles American business every 5 years, from the national to the local level. Core data include the number of establishments, annual sales, annual payroll and number of employees by type of business. Supplemental annual surveys are conducted periodically to provide more detailed information on a specific sectors, regions and topics.
- Description and data portal through American FactFinder: http://factfinder.census.gov/jsp/saff/SAFFInfo.jsp?_pageId=sp2_economic&_submenuId=
- Business data by county, metro area or zip code
Current Population Survey (CPS) http://www.census.gov/cps/
The Current Population Survey (CPS) is a monthly sample-based survey of about 50,000 households conducted by the Bureau of the Census for the Bureau of Labor Statistics. Focusing on individuals 16 years of age and over in the labor force, the CPS provides information on:
CPS data may also be accessed through the Department of Labor’s Bureau of Labor Statistics http://stats.bls.gov/cps/.
American Housing Survey (AHS) http://www.census.gov/hhes/www/housing/ahs/ahs.html
The American Housing Survey (AHS), conducted by the Bureau of the Census for the Department of Housing and Urban Development (HUD), collects data on housing types (e.g. multi- vs. single-family), size, vacancies, occupant income, housing costs, heating fuels, neighborhood quality, and recent movers. National data are collected in odd numbered years. Data for 47 selected Metropolitan Areas are collected about every six years. The national sample includes an average 55,000 housing units. Each metropolitan area sample includes at least 4,100 housing units. As of early 2008 the most recent metropolitan AHS data available are from 2004. The most recent data for some metropolitan areas dates back to 1998.
While the Census provides rich, detailed information, there are sometimes problems with using this data for HIA
- Information may be dated
The Census is conducted only once every 10 years. Especially towards the end of a 10-year cycle a population in a given area may have changed dramatically since the last census. The American Community Survey (ACS) was designed in part to correct this problem, but it is based on sampling and thus may not provide accurate estimates for small areas of interest to some HIAs.
While the Census Bureau puts great effort into counting each person, certain populations are likely to be undercounted. These tend to be exactly those populations who face high rates of ill health and health risks, including homeless, undocumented immigrants and the poor. See http://www.census.gov/dmd/www/techdoc1.html
- People commuting into or visiting the area are not counted
It is important to note that the Census counts residents. At the county level and for smaller areas, merely counting the number of residents is likely to underestimate the affected residents since people living elsewhere but working, going to school or utilizing services in the affected area may also be affected by the proposed policy or project. There are various ways to estimate the size of these other groups, but rarely do these other methods capture the detailed population information provided by the Census.
- Small area boundary issues
Similar to the above-mentioned problem of counting only residents, the effects of a project or policy may extend to people living outside the boundaries of a specified area, and at the same time such effects may not affect all individuals living within those boundaries. For instance, the economic and environmental impacts of a shopping mall located near a county boundary could actually be greater for residents of the neighboring county than for residents living in the county in which the mall is located. More generally this problem could be described as a mismatch between the actual impact boundaries of a project or policy and the administrative boundaries used by the Census (e.g. census blocks, census tracts, cities, zip code tabulation areas and counties) .
- Sampling vs. enumeration
While the Census attempts to count (i.e. enumerate) the entire population, many of the demographic and economic details about that population are estimates based on sampling. The Census collects basic demographic data from all households, and more detailed information from a 1-in-6 sample of households. This sampling frame is used to generally give reliable estimates at the census tract level. For smaller areas (e.g. blocks and block groups) and for detailed cross-tabulations, estimates based on sampled data may not be sufficiently robust.
Enumerated vs. Sampled Data Topics in the Census
100% (enumerated) data (Summary Files 1 & 2 )
Sampled data (Summary Files 3 & 4 )
For more on samping issues in the census see http://factfinder.census.gov/home/en/epss/sf3_compare.html and http://www.census.gov/prod/census2000/docs/sf3.pdf (Chapter 8, Accuracy of Data) for detail the Census.