Sample Design

The NLSY79 Child sample is comprised of all children born to NLSY79 mothers. Starting in 1986, the children of the NLSY79 mothers have been interviewed and assessed biennially to follow their cognitive, physical, and socio-emotional development. Starting in 1994, children who have reached the age of 15 by the end of the survey year are no longer assessed but instead complete personal interviews similar to those given to their mothers during late adolescence and into adulthood. Upon reaching age 15, the NLSY79 children become part of the NLSY79 Young Adult sample. Starting in 2016, NLSY79 children age 12 and older became part of the Young Adult sample.

As of 2018, a total of 11,551 children have been identified as having been born to the original 6,283 NLSY79 female respondents, mostly during the years that they have been interviewed. A modest number of children were born prior to 1979, the first main Youth interview round. An unknown number of additional children have been born to women of the dropped oversamples or women who have otherwise left the survey, subsequent to their attrition from the sample.

The number of children assessed during a given child survey year is a function of the number of children born to interviewed NLSY79 mothers, the number of children living in the homes of those mothers, and, finally, the number of those children actually interviewed. Of the 5,842 NLSY79 females eligible for the first child interview in 1986, more than 2,900 mothers and 4,971 children were interviewed. From this sample of eligible children, assessment data were collected for 4,786. As of the most recent survey, a total of 11,551 children have been identified as having been born to the original 6,283 NLSY79 female respondents. Of these, 4,354 children 12 and older were interviewed as young adults. Details on the sizes and eligibility criteria of the samples are discussed below.

• Sample Sizes: Who Was Interviewed in the Current Survey Round?
• Sample Changes over Time
• Sibling and Cousin Samples

Sample Sizes: Who Was Interviewed in the Current Survey Round?

In 2020, 4,354 young adults were interviewed (see Table 1), ranging in age from 12 to 50 as of the date of interview. For 2016 and earlier survey rounds, "interviewed" for children under age 15 means that some child-specific assessment information was obtained from either the mother or child in that survey year. For the Young Adult sample, a completion is defined as a case in which at least a certain amount of the Young Adult interview was completed.

NLSY79 mothers. Significant numbers of NLSY79 mothers have participated in the NLSY79 Child data collection effort over the years. (Users interested in the participation rates of NLSY79 mothers relative to nonmothers and other NLSY79 respondents will find details in the NLSY79 User's Guide.) Table 1 shows, for each Child survey, the proportion of women interviewed who are mothers. The table also indicates the number of children born to interviewed mothers. When appropriate weights are applied, NLSY79 women have had, on average, about 1.9 children, which is estimated to be their ultimate childbearing. While the childbearing for this cohort is now essentially completed, caution is still advised when generalizing from any selected portion of the child cohort.

Child Sample Eligibility. In the first round of the NLSY79 Child survey (1986), all children born to NLSY79 women (who were themselves interviewed) were eligible to be interviewed. Starting in 1988, children whose usual residence was outside the mother's household were excluded from the sample. This residence restriction, however, applies only to children who are not age-eligible for the Young Adult survey.

From 1986-1992, there was no upper age restriction on the Child sample. Starting in 1994, children who turned 15 by the end of the survey year became part of the Young Adult sample. In 2016, children who turned 12 by the end of the survey year are part of the Young Adult sample, and mothers completed the Mother Supplement for children age 13 or younger living in the household at least part-time. In 2018, children who turned 12 by the end of the survey year are part of the Young Adult sample, and no assessments from the Mother Supplement were administered.

Until 2008, the Child and Young Adult survey periods were restricted to a single calendar year. However, starting in 2010, the field period crossed over into the following calendar year, so the Child/Young Adult samples are more clearly distinguished based on year of birth. The table below indicates the birth year range for the younger Child cohort for 2010, 2012, 2014, and 2016:

Young Adult Sample Eligibility. Young Adult children who have at least one record in the child interview history are generally eligible for interview regardless of their residence status. In both 1994 and 1996, children of NLSY79 mothers who would be 15 or older by the end of the survey year were eligible to be interviewed as Young Adults. In the 1998 survey year, a cap was placed on the upper ages of the Young Adults, so that only those children 15 to 20 were interviewed as Young Adults. In 2000, the full sample of eligible Young Adults was again fielded, with no upper age limit imposed; however, the YAs between 15 and 20 from 38% of the black and Hispanic oversample families were not fielded in 2000 for budgetary reasons. These YAs were eligible again to be interviewed in 2002. For the 2004, 2006, and 2008 fieldings, there were also no sample restrictions for age or sample type. 

Beginning in 2010, Young Adult respondents over the age of 30 have been moved to a four-year interview cycle. Because of the structure of the Young Adult sample, in each survey round some respondents will be interviewed as Young Adults for the first time that round, some will have been last interviewed in the previous round, and some will have been last interviewed as Young Adults two (or more) rounds ago. NOTE: In the 1998 data collection only, the Young Adult sample was limited to respondents who were between the ages of 15 (by the end of the year) and 20 (at the date of interview).

Beginning in 2016, children ages 12 and over by the end of the year have been included in the Young Adult sample. Children age 14 answer the same questions as those ages 15 and 16. Children ages 12 and 13, while fielded as part of the Young Adult data collection, answer far fewer questions, and the Young Adult questionnaire has been modified to include some items from the Child Self-Administered Survey, answered by only this age group.

Sample Restrictions and Exclusions. Over the period of the survey the following adjustments have been made to the Child and Young Adult samples:

• 1990. Following the 1990 interview, none of the 1,643 members of the economically disadvantaged, nonblack/non-Hispanic NLSY79 main Youth sample were eligible for interview. In anticipation of the deletion of this sample, the children of the mothers in this subsample were excluded beginning with the 1990 interview and were not assessed or interviewed as Young Adults in any subsequent round. The sample nonetheless retains sufficient numbers of children from this category to maintain its full national representation. 
• 1994. Starting in 1994, with the introduction of the Young Adult surveys, children age 15 and older become part of the young adult sample and are eligible for interview regardless of residence.  This means that, starting with the 1994 survey, the NLSY79 younger child sample was redefined as children under age 15 as of the end of the survey year.
• 1994-1998. Young Adult sample selection was limited to children 15 and older as of December 31 of the survey year who were either assessed or living with their mother in either of the two previous rounds.
• 1998. In the 1998 survey year, a cap was placed on the upper ages of the young adult sample, so that only those children 15 to 20 were interviewed as Young Adults. 
• 2000. Sample selection for the Young Adult was widened to included children 15 and older by December 31 of the survey year who had at least some assessment history. YAs who had been age-eligible but not fielded in 1994 through 1998 and had some assessments were brought into the YA sample at that point.
• 2000. In 2000 the criteria for both younger children and young adults under age 21 were restricted (for that survey round only) to exclude the younger children (0-14) and young adults (15-20) from a random sample of 38 percent the black and Hispanic oversample mothers. This restriction means that while the full set of oversample mothers was contacted in 2000, only about 60 percent of their children under age 21 were part of the fielded sample targeted for interview. In 2002, the oversample cases that were excluded in 2000 were restored to the fielded sample eligible for interview. A flag in the database (C00115.13, CEXCLUDED2000) indicates which children under age 21 were part of the excluded oversample in 2000.
• 2010 onward. Beginning in 2010, young adults over age 30 have only been interviewed every four years. The interviewed sample is selected by age as of December 31 of the survey year, so that approximately half of the older young adults have been eligible each round. Since 2010, young adults age 31-32, 35-36, 39- 40, 42-44, etc. as of December 31 of the target year were not be fielded.
• 2016. Beginning in 2016, the Child Supplement and the Child Self-Administered supplement were no longer administered, and the children ages 12 to 14 as of December 31 of the survey year were fielded as part of the Young Adult. The Mother Supplement was administered to interviewed mothers with children ages 13 and under living with them at least part time.
• 2018. In 2018, children ages 12 to 14 as of December 31 of the survey year were fielded as part of the Young Adult survey, but no Mother Supplement assessments were completed. While NLSY79 mothers did answer a small number of child health and schooling questions traditionally in the Mother Supplement for children age 18 and younger, no sampling weight is provided for children under the age of 11, and children age 12 and over will only have a sampling weight if interviewed as a Young Adult.
• 2020. In 2020, children ages 12 and up were fielded as part of the Young Adult survey, and NLSY79 mothers answered only questions about the usual residence of and contact with their children.

Sample Changes over Time

Age shift in the child samples. The age distribution of interviewed women in 2020 (between 56 to 63 as of December 31, 2020) underscores the fact that most NLSY79 women have reached the end of their childbearing years. Recent survey rounds also mark a continuing shift in the age composition of the child sample from a predominantly younger child group to a more young adult population. As of the current survey round, about 99 percent of the interviewed child sample is age 15 or older and about 95% are age 21 and over. Since very few, if any, children remain to be born in forthcoming NLSY79 survey rounds, a rapid transition towards an even older child population is evident, with the majority of the children in their 20s, 30s, and 40s. Paralleling this shift, the younger component of the overall sample increasingly resides in middle class households and were born to women at older ages. This trend implies that users should exercise caution in undertaking within-sample analyses in which comparisons are made between children at different ages.

Table 2 shows child's year of birth by mother's age at birth of child for all NLSY79 children. Whereas at one time a large proportion of NLSY79 children had been born to adolescent mothers, nearly all of the children and young adults in the current sample had been born to women age 20 and over. In the current round, about 9% of interviewed young adults were born to adolescent mothers. 

Sample limitations. Table 2 suggests one other caveat for studies that focus on the consequences of earlier, adolescent childbearing for this cohort of women. A modest proportion of the children ((311 of the 4,354 interviewed in the current survey) were born prior to the first NLSY79 interview round. If essential explanatory inputs for analysis include pre-1979 points (e.g., employment status in 1977 or early paternal presence in the home), sample size may be temporally constrained because of this left-censoring problem--the unavailability of some data elements for the pre-survey period. All such cases fall in the upper young adult ages, and could affect analyses for young adult children born in 1978 or earlier.

The increasing heterogeneity of the child sample may also be noted in other ways from Table 2. While there has been an increase in sample heterogeneity over the years, users should remain mindful that the oldest and youngest children in the sample are likely to come from families that differ in their socio-economic characteristics. However, it is also becoming increasingly reasonable to generalize from the NLSY79 sample of children to broader representations of selected U.S. child populations; overall, it is worth reiterating that as of this date, the cohort of women have completed essentially all of their childbearing. 

Sample Changes over Time. The increasing heterogeneity of the child sample over time may be noted from Table 3. This table summarizes the age mix as well as the race/ethnic mix of the child sample as it moves forward in time from 1986, the first year of the child interviews, to the current survey round. Over time, there is a gradual transition towards an older average age at interview. Notwithstanding this increase in age, the overall sample has changed very little over time in its racial and ethnic makeup. There has been some counterbalancing between higher minority birthrates and the reality that a higher proportion of the births in recent years are to older, white women. As evident in Table 3, sample sizes have varied over the years, largely reflecting the variations in data collection already noted. The single largest cause of decline from 1988 to 1990 was the removal of the economically disadvantaged white oversample. The slight decline from 1996 to 1998 was related to the capping of interviews in that year at age 20; and the decline from 1998 to 2000 reflected the one-time exclusion of a part of the black and Hispanic oversamples. In the 2002 interview round, there is no age or other sample exclusion, which accounts for the increase in sample size between 2000 and 2002. This increase is most evident in the young adult ages as the larger age cohorts continue to shift from child to young adult.

Changes in the Young Adult Sample. When the Young Adult Survey was first fielded in 1994, 1,111 older children were identified as eligible to be fielded, and interviews were conducted with 979 Young Adults. The Young Adults interviewed in the 1994 survey round were disproportionately Black or Hispanic and born to younger mothers. In 1998 only, an additional age restriction was imposed: respondents had to be under 21 by the date of interview. A decision was made to retain data from the 15 Young Adults who were inadvertently interviewed even though they had already turned 21. In 2000, approximately 40 percent of black and Hispanic oversample cases between age 15 and 20 were not fielded. Beginning in 2010, respondents over the age of 30 are interviewed every four years.  (The interviewed sample is selected by age as of December 31 of the survey year, so that approximately half of the older young adults are eligible each round. Since 2010, young adults age 31-32, 35-36, 39-40, 43-44, etc. as of December 31 of the target year are not fielded). Table 4 shows the sample of Young Adults interviewed in 2020.

Sibling and Cousin Samples

When the sample selection for NLSY79 was made, all individuals living in the selected households who were between the ages of 14 and 21 on December 31, 1978, were selected for sample inclusion. In many instances, siblings were included in the original sample. This has methodological implications for those who are concerned about the lack of complete independence between all of the NLSY79 cases. There are many main Youth sisters who are respondents in the NLSY79 sample. The Sample Design & Screening Process section of the NLSY79 User's Guide details this information, so it is not explained extensively here. 

The focus in this section is on the children who have been born to the female respondents in this kinship sample. From the child's perspective, children of sisters are cousins to each other. Over the course of the survey years, more than 3,000 children in the sample have been identified as having an aunt in the NLSY79 main youth sample. Most of these children have one aunt, but smaller numbers have multiple aunts. While the number of children who are cousins is considerable, the precise numbers available for a particular research project are contingent on the objectives of the research. For example, will the researcher limit the sample to children or women interviewed in only the current survey year or will the researcher include mothers or children interviewed in one or more of the earlier survey rounds?

More typically, researchers utilize the large number of child sibling sample cases that have been born to the female respondents. As seen in Table 5, most of the women have had more than one child, including a rather large sample of women who have had three or more children, as they approach the end of their childbearing years.

NLSY79 female respondents who are sisters, as well as children born to those sisters, can be readily identified. NLSY79 female respondents who are sisters and who were resident in the same household when the original main youth sample was selected can be identified by variables on the child file called SISTID1- SISTID3 (C00010.00-C00012.00). Children born to a particular respondent all share the same stem as the ID of their mother, with an additional two-digit identifier (01, 02 etc.) that typically (although not in all cases) clarifies their sibling placement.

In addition to multiple births, there are many family units where two or more children are widely spaced in age, thus enhancing the possibility of exploring the impact of childbearing on children that have been born to the same mother at different maternal life cycle stages. Larger sample sizes can be generated by incorporating women who were not interviewed in the current survey year but who had been interviewed in earlier survey rounds.

Depending on research topic, some users may be interested in young adults with siblings in the young adult sample. Sample sizes for this subset are shown in Table 6. Most of these siblings have also been interviewed in past rounds, providing researchers with ample data to carry out within-family analyses.