The documentation for the public use samples prior to 1981 does not include an explicit definition of what constitutes a family, though the Field Representative's Manual for some years includes examples of "a typical family group" (i.e., "husband, wife and unmarried children, or a parent and child, two or more sisters or some similar clear-cut arrangement").

The 1981 Field Representative's Manual included the following definition of a "Family Unit": "This includes all household members related by blood, marriage, or adoption, and includes 'foster' persons; e.g., head, wife of head, children (unmarried, married and their spouse, step, foster, wards, adoptive), grandchildren, aunts, uncles, nephews, nieces, etc. This does not include unrelated household members. These persons make up their own family unit." While the final two sentences of this definition seem to imply that an unrelated individual constituted a separate family unit, in practice, the 1981 public use files continued to code unrelated individuals as "0" in FAMSIZE.

The Field Representative's Manual for 1982 and several subsequent years defined a family as "a group of two or more related persons who are living together - same household; for example, the reference person, his/her spouse, foster son, daughter, son-in-law, and their children, and the wife's uncle." The definition continued, "Additional groups of persons living in the household who are related to each other but not to the reference person are considered to be separate families; for example, a lodger and his/her family, a household employee and his/her spouse. Hence, there may be more than one family living in a household." Thus, the documentation text defined a family as unit of two or more persons, while coding practices for FAMSIZE implicitly treated an unrelated individual as a family with a FAMSIZE of 1.

The Field Representative's Manual defined a family as "an individual or a group of two or more related persons who are living together in the same household; for example, the reference person, his/her spouse, foster son, daughter, son-in-law, and their children, and the wife's uncle. Also, unmarried couples (same-sex and opposite-sex couples) are considered as belonging to the same family. Additional groups of persons living in the household who are related to each other, but not to the reference person, are considered to be separate families; for example, a lodger and his/her family, or a household employee and his/her spouse, or a single boarder with no one related to him/her living in the household. Hence, there may be more than one family living in a household, and a family can consist of only one person."

Noteworthy changes here include both the acknowledgement of one-person families and the recognition of family relationship ties other than blood, marriage, and adoption (in the case of unmarried same-sex and opposite-sex couples). The Field Representative's Manual for most subsequent years reproduced the same text. The 2002 Field Representative's Manual employed slightly different wording ("A Family can be a single person or a group of two or more related persons living in the same household; for example, the reference person, his/her spouse (or unmarried partner), foster son, daughter, son-in-law, and their children, and the wife's uncle") but had essentially the same meaning.

For 1969-1972, FAMSIZE of 1 means (relative to the variable for family relationships, or FAMREL) either: (a) Head of family or unrelated individual not living alone or (b) Wife (husband living at home and in the Armed Forces).

For 1973-1981, FAMSIZE of 1 means (relative to FAMREL) only "Head of family or unrelated individual not living alone."

For 1982-1994, FAMSIZE of 1 means (relative to FAMREL) either (a) "Reference person, living alone," or (b) "Reference person, 2+ persons in household," or (c) "Child of ineligible reference person."

In 1995, FAMSIZE of 1 means (relative to FAMREL) either (a) "Reference person, living alone," or (b) "Reference person, 2 or more persons in household," or (c) "Spouse, other spouse in Armed Forces and living at home," or (d) "Child of military family with no eligible reference person."

In 1996, FAMSIZE of 1 means (relative to FAMREL) either (a) "Reference person, living alone," or (b) "Reference person, 2+ persons in household," or (c) "Child of military family with no eligible reference person."

Beginning in 1997, FAMSIZE seems to have an unequivocal meaning, referring to the number of people in the family (where, by definition, families for 1997 forward may have 1 or more members).