From the U.S. Department of Health and Human Services, Health Resources & Services Administration, Definition of Family: “A family is a group of two or more persons related by birth, marriage, or adoption who live together; all such related persons are considered as members of one family. For instance, if an older married couple, their daughter and her husband and two children, and the older couple's nephew all lived in the same house or apartment; they would all be considered members of a single family.”
Source: 2021 American Community Survey (ACS), 1-year estimates.
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From Topics > Families & Living Arrangements:
From the Census Library / Publications and Working Papers:
From the Newsroom / Stats for Stories (SFS):
From Population Publications > Current Population Reports (CPS):