The Census Bureau asks some questions of everyone and about every housing unit. The information derived from these questions is called 100-percent data. It asks other questions of a sample of the population. The size of the sample varies from one area to another, but overall it is about one household out of every six for the United States. Sampling is done to lower costs and reduce the burden of responding on the public.
Geographic coverage is an important consideration when trying to find census data.
Each statistical table covers one or more specified geographical entity. The Census Bureau furnishes data for legal governmental units, such as states, counties, and cities, etc. The Census Bureau organizes geographic entities into hierarchies for tabulating and reporting statistics. Generally the larger the geographic entity, the more detailed the statistics that can be reported. Tracts and BNA's are large enough in population to permit the Bureas to publish extensive 1990 sample data.