The Census Bureau collects and publishes a variety of statistics about the people of the United States in two basic ways: the constitutionally required decennial census, taken in years ending in "0," that counts the population, and in surveys and other periodic programs that estimate data between the censuses. On the authorization of the appropriate local governments, the Bureau also conducts special censuses on a cost-reimbursable basis.

Title 13 of the United States Code, the statute under which the Census Bureau operates, authorizes the population censuses and surveys, and makes public compliance with the censuses mandatory. Response to the surveys is voluntary. In all cases, the law strictly protects the confidentiality of the information collected about individuals and the households and other quarters in which they live, and prescribes heavy penalties if Bureau employees disclose such information.

There have been decennial censuses ever since 1790. Certain questions have been asked for all persons (for 1990, see the" 100-percent" items in fig. 1) to provide precise data for units of analysis as small as a city block. Since 1940, other questions have been asked for a fraction (sample) of persons or households; the resulting data are reported as "weighted" or "estimates" rather than 100-percent counts and are published for areas as small as block groups (BG's; see below). BG's in the summary tape files (see p. 3) average about 700 people. Figure 1 shows the sample items for 1990.

The census also asked a number of 100-percent and sample questions about the housing in which the people lived, so that some of the census reports show population characteristics cross-tabulated by housing characteristics, such as renter-occupied housing units by the occupants' race (for details on the housing questions, see Factfinder CFF 6, "Housing Statistics"), and "derived" statistics like poverty status or family type.