4.3.1 Population Data
Each of the approaches considered requires some knowledge about the size and demographic characteristics of populations living close to a nuclear facility, and this information must be on a suitable time scale. The committee is convinced that the information should be for geographic areas smaller, perhaps much smaller, than counties.
Population counts for small areas are available from the U.S. Census.15 Every 10 years, in years ending in “0,” the Bureau performs the official count of people living in the United States. The Bureau of the Census supplements the decennial census on a continuing basis by the sample surveys and statistical models that make up the American Community Survey (ACS16), which provides more data on social and economic characteristics than does the decennial census. The ACS sends surveys to approximately 3 million housing units and group quarters in the United States in every county, so detailed information on a small geographic scale may be sparse. In 2009, completed ACS interviews represented 66.2 percent of the housing units initially selected for inclusion in the sample.
The decennial census reports show aggregate population demographic data for a standard set of geographic regions defined by state, county, census tract, block group, and block. Blocks are small geographic areas bounded by visible features such as streets and railroad tracks and by nonvisible boundaries such as property lines or county boundaries. Block groups consist of collections of blocks and are typically defined to contain 600 to 3,000 people. Census tracts contain several block groups and typically contain 1,200 to 8,000 people (with a target of 4,000 people) (www.census.gov). While the typical and target population sizes generally hold, there is wide variation across the country and some tracts contain population counts well below or above the example ranges stated here. The spatial size of the census tract also varies widely across the country. Census tracts were not fully defined until the 1980 Census. The 1970 Census had tracts for some areas, but not the entire country. Enumeration units at one level do not cross those at higher levels so, for instance, a census-tract boundary does not cross a county boundary. This nested hierarchy ensures that counts are “upward compatible.” County boundaries rarely change over time, and state boundaries do not change at all. If an analysis requires attention to these changes, the Geography Division of the Bureau of the Census may be able to help.
Census Summary File data from each household include information