Two or more persons, including the householder, who are related by birth, marriage, or adoption, and who live together as one household; all such persons are considered as members of one family. (Persons not in families and not inmates of institutions are classified as unrelated individuals.) Families are defined using responses to the complete-count household relationship question.

If the son/daughter of the person or couple who maintains the household and the son's or daughter's spouse and /or children are members of the household, they are treated as part of the householder's family. 1 roomer/boarder and his/her spouse who are not related to the person or persons who maintain the household, or a resident employee and his/her spouse living in are not counted as a family, but as individuals unrelated to the householder. Thus, a household can contain only one family for purposes of census tabulations.

A person maintaining a household alone, or two or more unrelated persons are regarded as a household but not as a family. Thus, some households do not contain a family.

(In certain Census Bureau surveys, families as defined here are referred to as "primary families." The term "secondary family" refers to a resident family unrelated to the householder, such as a roomer and his/her spouse. Tabulations of families from such surveys include secondary families.)