of extant studies, to exploit new opportunities for study of small geographic areas, and to examine temporal changes.
Much of our knowledge of social transformations over recent decades has been obtained from analyses of census data. The decennial census has developed into an indispensable tool of government and a servant of many other purposes. A small number of questions deemed suitable and necessary for the government to ask has sufficed to delineate many of the most important features of the nation's peoples and their principal activities. Age, sex, race, ethnicity, family relationship, parenthood, place of current and previous residence, birthplace, education, employment, income—these topics covered in the census provide raw data for specification and analysis of each of the social and economic transformations mentioned above, and for an endless array of reports and research by public and private agencies.
The census of 1940 was the first to go beyond the traditional complete enumeration and employ sampling for some questions. Sampling continued in subsequent censuses, with a roughly 1-in-8 sample used in the 1990 census for most census data. Development of sampling theory and methods and the need for much more frequent temporal detail on many topics gave rise to a wide variety of federal, state, and privately sponsored surveys from the 1940s to the present. These surveys have not replaced the need for the census to provide data for small geographic areas, for small and widely dispersed population groups (such as American Indians), and for a benchmark for intercensal surveys.
The Census Bureau releases decennial census data in two forms. One form is aggregated data for geographic units and administrative jurisdictions. The aggregated data are for varying geographic sizes ranging from city blocks to metropolitan statistical areas (MSAs) to states. Aggregated data are made available in published form that are widely available in libraries and research centers and in summary tapes. The Census Bureau released a number of different summary tape files containing an immense quantity of census data arranged by geographic category. The data consist of detailed tabulations for each geographic area and tables by conventional categories, such as age and race. A typical table might show the population by age, sex, and race for census tracts for a city. Such a table would allow an educational planner to note the elementary school-age population for Asian children for small areas for purposes of planning the needs for bilingual language instruction. With the aid of the Census Bureau's TIGER software for geographical referencing, aggregated data may be further combined to any collection of geographic units, including local service areas,