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Overview of Subfamilies and Family Interrelationships

(Back to Family Interrelationships )

What are subfamilies, and how does the IPUMS measure them?

Subfamilies are simply families that live in the household of someone else. The Census Bureau defines two main types of subfamilies. Married-couple subfamilies comprise a husband and wife living with or without children. Parent-child subfamilies comprise an unmarried parent living with one or more of their own, never-married children under 18 years of age. Subfamilies can be further divided into related subfamilies (whose members are related to the household head by birth, marriage, or adoption) and unrelated subfamilies (whose members are not related to the household head). Unrelated subfamilies (which used to be called secondary families) are no longer measured by the Census Bureau, but the IPUMS includes them for users' convenience.

The IPUMS provides four variables concerning subfamilies:

Name	Type	Description	Non-subfamily analogue	Description
SFTYPE	Person	Type of subfamily (if any) to which each person belongs	---
SFRELATE	Person	Relationship within subfamily (if any)	---
SUBFAM	Person	Identifies the subfamily (if any) to which each person belongs	FAMUNIT	Identifies the family to which each person belongs
NSUBFAM	Household	Number of subfamilies in household	NFAMS	Number of families in household

For more information, see the individual variable descriptions.

The following table gives examples of these variables in two hypothetical households:

Example and person number	Relationship to head	Spouse location	Mother location	Father location	Age	Sex	Marital Status	SFTYPE (Subfamily type)	SFRELATE (Relationship within subfamily)	SUBFAM (Subfamily number)

Example 1
1	Head	2	0	0	50	M	Married	0	0	0
2	Spouse	1	0	0	49	F	Married	0	0	0
3	Child	4	2	1	28	M	Married	2	1	1
4	Child-in-law	3	0	0	29	F	Married	2	2	1
5	Sibling-in-law	0	0	0	40	F	Never married	4	1	2
6	Other relative	0	5	0	12	M	Never married	4	3	2
7	Non-relative	8	0	0	35	M	Married	5	1	3
8	Non-relative	7	0	0	33	F	Married	5	2	3
9	Non-relative	0	8	7	5	F	Never married	5	3	3
10	Non-relative	0	0	0	22	M	Never married	0	0	0
11	Non-relative	0	0	0	21	M	Never married	0	0	0
.	.	. .	.	.	.	.	. .	.	.	.
Example 2	.	.	.	.	. .	.	.	.	.	.
1	Head	0	2	0	26	F	Never married	0	0	0
2	Parent	0	0	0	55	M	Divorced	3	1	1
3	Sibling	0	0	2	19	F	Never married	0	0	0
4	Sibling	0	0	2	17	F	Never married	3	3	1
5	Non-relative	0	0	0	29	M	Separated	7	1	2
6	Non-relative	0	0	5	4	M	Never married	7	3	2
7	Non-relative	0	0	0	50	F	Never married	0	0	0
8	Non-relative	0	0	0	42	F	Never married	0	0	0
9	Non-relative	0	0	0	36	M	Never married	0	0	0

In Example 1:

Persons 3 and 4 are relatives of the head and are married, but do not have any children in the household. They are therefore classified as a married-couple related subfamily without children (SFTYPE = 2) and have been coded as the first subfamily in the household (SUBFAM = 1), with the husband as the reference person (SFRELATE = 1) and the wife as the spouse (SFRELATE = 2).
Person 5 is also a relative of the head and has a child (person 6) who is under 18 and has never been married. They are therefore classified as a mother-child related subfamily (SFTYPE = 4) and have been coded as the second subfamily in the household (SUBFAM = 2), with the mother as the reference person (SFRELATE = 1) and her son as the child (SFRELATE = 3).
Persons 7 and 8 are not relatives of the head, but they are married and have a child (person 9) who is under 18 and has never been married. They therefore constitute a married-couple unrelated subfamily with children (SFTYPE = 5) and have been coded as the third subfamily in the household (SUBFAM = 3), with the relationships (SFRELATE) as described above.

In Example 2:

Person 2 is the father of the head and has a child (person 4) who is under 18 and has never been married. The two are thus classified as a father-child related subfamily (SFTYPE = 3), the first subfamily in the household (SUBFAM = 1). Note that person 3 is also the child of person 2, but is at least 18 years old and is not included in the subfamily.
Person 5, who is unrelated to the head, is the father of person 6. They constitute a father-child unrelated subfamily (SFTYPE = 7), the second subfamily in the household (SUBFAM = 2).

Finally, note that all members of the household who are not members of a subfamily are coded as 0 for all subfamily variables, whether or not they are related to the head.

How do subfamilies differ from the "families" that the IPUMS already measures?

SUBFAM is analogous to FAMUNIT in that it identifies membership in unique family units within each household, but the specific related unit measured by each is different.

FAMUNIT groups all members of households into extended family units. The household head and all relatives are coded 1. Non-relatives are grouped into family units when possible; otherwise, each non-relative is assigned a unique FAMUNIT code.

In SUBFAM, household heads are never in subfamilies and are always coded as 0. The head's relatives may be grouped into subfamilies with one another as described above, but are never grouped with non-relatives. Non-relatives who are related to other non-relatives can be grouped into subfamilies with one another. Non-relatives who are not related to anyone in the household are not in subfamilies and are coded as 0.

In example 1, persons 1-6 are thus included as members of the head's family (FAMUNIT = 1), even though this group also includes two subfamilies. The same is true for persons 1-4 in example 2, which includes one subfamily. Additionally, FAMUNIT counts as separate family units all individuals who are unrelated to the head and who live without a spouse or children (persons 10 and 11 in example 1; persons 7-9 in example 2); SUBFAM does not and codes them as 0. However, members of all unrelated subfamilies (persons 7-9 in example 1; persons 5 and 6 in example 2) receive unique codes in both FAMUNIT and SUBFAM.

This difference carries over to NSUBFAM (the total number of subfamilies in the household) and NFAMS (the total number of families in the household). Example 1 includes three subfamilies (so NSUBFAM = 3), and example 2 includes two subfamilies (NSUBFAM = 2). NFAMS is more complicated. Example 1 includes one group of people related to the head, one unrelated subfamily, and two unrelated individuals, for a total NFAMS of 4. Example 2 includes one group of people related to the head, one unrelated subfamily, and three unrelated individuals, for a total NFAMS of 5.

Example and person number	Relationship to head	Spouse location	Mother location	Father location	Age	Sex	Marital Status	SUBFAM (Subfamily number)	FAMUNIT (Family number)	NSUBFAM (Number of subfamilies)	NFAMS (Number of families)

Example 1
1	Head	2	0	0	50	M	Married	0	1	3	4
2	Spouse	1	0	0	49	F	Married	0	1	3	4
3	Child	4	2	1	28	M	Married	1	1	3	4
4	Child-in-law	3	0	0	29	F	Married	1	1	3	4
5	Sibling-in-law	0	0	0	40	F	Never married	2	1	3	4
6	Other relative	0	5	0	12	M	Never married	2	1	3	4
7	Non-relative	8	0	0	35	M	Married	3	2	3	4
8	Non-relative	7	0	0	33	F	Married	3	2	3	4
9	Non-relative	0	8	7	5	F	Never married	3	2	3	4
10	Non-relative	0	0	0	22	M	Never married	0	3	3	4
11	Non-relative	0	0	0	21	M	Never married	0	4	3	4
.	.	.	.	.	.	.	.	.	.	.	.
Example 2	.	.	.	.	.	.	.	.	.	.	.
1	Head	0	2	0	26	F	Never married	0	1	2	5
2	Parent	0	0	0	55	M	Divorced	1	1	2	5
3	Sibling	0	0	2	19	F	Never married	0	1	2	5
4	Sibling	0	0	2	17	F	Never married	1	1	2	5
5	Non-relative	0	0	0	29	M	Separated	2	2	2	5
6	Non-relative	0	0	5	4	M	Never married	2	2	2	5
7	Non-relative	0	0	0	50	F	Never married	0	3	2	5
8	Non-relative	0	0	0	42	F	Never married	0	4	2	5
9	Non-relative	0	0	0	36	M	Never married	0	5	2	5

What weights should be used when studying subfamilies?

Complex household structures naturally create questions about what weights to use. In general, the IPUMS advises the following:

Use the PERWT of individuals when studying the characteristics of individuals within subfamilies. Examples include the ages of children in subfamilies or the incomes of wives in married-couple subfamilies.
Use the PERWT of the subfamily's reference person (SFRELATE = 1) when studying the characteristics of subfamilies themselves. Examples include the total number of parent-child subfamilies or the average total income of subfamilies.
Use the HHWT of the household when studying the characteristics of household containing subfamilies. Examples include the proportion of households containing subfamilies or the average number of people in households containing subfamilies.

What else should I know about subfamily measurement?

Users should note that the IPUMS subfamily variables are not based on the original Census Bureau subfamily variables, which are often inaccurate and difficult to harmonize across time (see this article for more on problems with Census Bureau subfamilies). Instead, IPUMS subfamilies are based on the IPUMS family interrelationship variables SPLOC, MOMLOC, and POPLOC. The extra information contained in these variables considerably improves the accuracy of subfamily identification

Users should also be aware that the IPUMS does not link unmarried partners as spouses. This means that many people who could be considered part of the head's family (if they are a partner of the head) or part of a married-couple subfamily (if they are a partner of another household member) will be classified instead as an unrelated individual (if they do not have children living with them) or as the reference person in a parent-child subfamily (if they do have children living with them).

IPUMS plans to link unmarried partners as spouses in the future; until then, users should be aware that the IPUMS subfamily variables may overestimate the prevalence of parent-child subfamilies.

In a small number of households, subfamilies will overlap. For example, there will be a parent-child subfamily in which the child is also the parent to another child. Rather than coding these as two separate subfamilies, the IPUMS classifies all these individuals as a single parent-child subfamily.

Finally, the new subfamily variables do not apply to group-quarters cases.

(Back to Family Interrelationships )