Before discussing the history of census data on households and related characteristics, it is helpful to provide some general concepts used currently. A housing unit is a physical structure (e.g., a house or an apartment) intended for occupancy as separate living quarters by one or more persons (e.g., a family or unrelated persons or a person living alone). Housing units are either occupied or vacant, and occupied housing units are either owned (with or without a mortgage) or rented. Persons living in a housing unit constitute a household, and the person or one of the persons in whose name the housing unit is owned or rented is defined as the householder. Households are defined as family households or nonfamily households. Family households are those households with one or more persons related to the householder by blood, marriage, or adoption. All other households, including one-person households, are nonfamily households. A small proportion of the population lives in group quarters (including institutionalized group quarters such as correctional institutions and nursing homes and noninstitutionalized group quarters such as college dormitories and military barracks. The group quarters population, which represented 2.6 percent of the total U.S. population in 2010, is not covered in this chapter.
While information in the decennial census of population has been collected primarily by household since 1790, little information on households was published in census volumes until 1940, and a question on relationship to householder was first asked in the 1880 census. Data on the number of households by size for 1790 are available from an analysis of census schedules published in U.S. Bureau of the Census (1909), A Century of Population Growth from the First Census of the United States to the Twelfth: 1790 to 1900. Starting with 1850, when census schedules first listed each person in a household by name, estimates of data on households and relationship to householder are available based on the Integrated Public Use Microdata Series (IPUMS), as shown in Steven Ruggles (2006), “Family and Household Composition,” Historical Statistics of the United States: Millennial Edition, Vol. 1, pp. 1-660 to 1-689. (See Minnesota Population Center, no date, “IPUMS-USA, Frequently Asked Questions (FAQ)“)
There have been many changes over time in definitions of households and related characteristics. In addition to the sources cited above, see Frank Hobbs, and Nicole Stoops (2002), “Demographic Trends in the 20th Century,” U.S. Census Bureau, Census 2000 Special Reports, CENSR-4, and Data Sources.