A subfamily is a married couple with or without own children under 18 years old who are never-married, or a single parent with one or more own never-married children under 18 years old. A subfamily does not maintain their own household, but lives in a household where the householder or householders spouse is a relative. Subfamilies are defined during processing of sample data.

In some labor force tabulations, both one-parent families and one-parent subfamilies are included in the total number of children living with one parent, while both married-couple families and married-couple subfamilies are included in the total number of children living with two parents.

Own child is a never-married child under 18 years who is a son or daughter of the householder by birth, marriage (a stepchild), or adoption. For 100-percent tabulations, 'own children' consist of all sons/daughters of householders who are under 18 years old. For sample data, 'own children' consist of sons/daughters of householders who are under 18 years old and who have never been married. Therefore, numbers of own children of householders may be different in these two tabulations since marital status was not collected as a 100-percent item in Census 2000.

In certain tabulations, own children are further classified as living with two parents or with one parent only. Own children living with two parents are by definition found only in married-couple families. In a subfamily, an "own child" is a child under 18 years old who is a natural-born child, stepchild, or an adopted child of a mother in a mother-child subfamily, a father in father-child subfamily, or either spouse in a married-couple subfamily. (Note: In the tabulation under "EMPLOYMENT STATUS" of own children under 6 years by employment status of parents, the number of "own children" includes any child under 6 years old in a family or a subfamily who is a son or daughter, by birth, marriage, or adoption, of a member of the householder's family, but not necessarily of the householder.)