The result of a decennial census is a collection of data products, which can generally be classified into two broad categories: basic population counts and summaries of the characteristics of areas or groups.1 Collectively, these data products are used for a wide variety of public and private needs; they are examined in a myriad of contexts and interpreted in many different ways.2 Proper evaluation of a census demands assessment of the quality and usefulness of its results in the context in which those results are actually used.

The first type of census data product—population counts for the nation as a whole and for subnational areas—satisfies the constitutional mandate for the census by providing the state-level counts required to reapportion the U.S. House of Representatives. Likewise, small-area population counts are the essential building blocks used for redistricting within states and localities. Basic population counts, sometimes differentiated by demographic group, are crucial for a variety of other uses, including:

  • calibration of data from other collection and survey programs, such as the Current Population Survey, the Vital Statistics of the United States, and the Uniform Crime Reports;

  • determination of eligibility for federal and state government funding programs;

  • comparison and ranking of areas (such as cities and metropolitan areas) for such purposes as advertising, marketing, and public information; and

  • benchmarking of intercensal population estimates.

Census count data are used to estimate both the level (raw count) and the share (proportion) of total population across different geographic areas; they are also used to compute change over time for either levels or shares.

The second type of census data product—local area or group characteristics—includes the various counts and averages that result from detailed cross-classification by geographic, demographic, and socioeconomic variables, particularly those collected on the decennial census long form. Examples of these characteristics include per capita income by census tract and state-level counts by demographic group, educational level, and employment type. Data sets of this type form the cornerstone of basic and applied socioeconomic research.


A third type of data product not considered in this categorization are public-use microdata sample (PUMS) files, which are sampled individual records from census files (using appropriate safeguards to protect confidentiality and privacy); these files are used to support a wide range of academic research.


See National Research Council (1985:Ch.2) and National Research Council (1995:Ch.1, Apps. C, D, E, F, G, H, and M) for more detailed discussions of census data use.

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