Sample Design

Sample Design

Sample Changes over Time

Age shift in the child samples. The age distribution of interviewed women in 2014 (between 49 to 58) 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 95 percent of the interviewed child sample is age 15 or older (the "young adults") and about 80% 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 and 30s. 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, the entire younger child sample was born to women ages 35 and older and about 12% of interviewed young adults were born to adolescent mothers.

Table 2. Child's Birth Year by Age of Mother at Birth of Child: All NLSY79 Children

 Year of Birth Age of Mother at Birth of Child
LT 17 17-19 20-22 23-25 26-28 29-31 32-34 35-37 38-40 41-43 44+ Total
Before 1979 353 702 166 0 1 0 0 0 0 0 0 1222
1979 55 205 268 0 0 0 0 0 0 0 0 528
1980 39 228 296 44 0 0 0 0 0 0 0 607
1981 11 213 342 148 0 0 0 0 0 0 0 714
1982 0 165 293 245 0 0 0 0 0 0 0 703
1983 0 105 263 303 44 0 0 0 0 0 0 715
1984 0 32 233 271 120 0 0 0 0 0 0 656
1985 0 0 226 256 194 0 0 0 0 0 0 676
1986 0 0 99 237 231 48 0 0 0 0 0 615
1987 0 0 23 248 251 100 0 0 0 0 0 622
1988 0 0 0 206 222 154 0 0 0 0 0 582
1989 0 0 0 115 274 210 30 0 0 0 0 629
1990 0 0 0 26 227 177 65 0 0 0 0 495
1991 0 0 0 0 175 163 84 0 0 0 0 422
1992 0 0 0 0 84 194 108 14 0 0 0 400
1993 0 0 0 0 29 171 111 38 0 0 0 349
1994 0 0 0 0 0 114 130 52 0 0 0 296
1995 0 0 0 0 0 58 114 64 7 0 0 243
1996 0 0 0 0 0 25 125 66 13 0 0 229
1997 0 0 0 0 0 0 104 90 19 0 0 213
1998 0 0 0 0 0 0 35 84 35 1 0 155
1999 0 0 0 0 0 0 14 85 22 6 0 127
2000 0 0 0 0 0 0 0 54 34 10 0 98
2001 0 0 0 0 0 0 0 30 38 13 0 81
2002 0 0 0 0 0 0 0 2 32  9 2 45
2003 0 0 0 0 0 0 0 0 20 13 0 33
2004 0 0 0 0 0 0 0 0 11 9 3 23
2005 0 0 0 0 0 0 0 0 3 10 2 15
2006 0 0 0 0 0 0 0 0 0 4 7 11
2007 0 0 0 0 0 0 0 0 0 1 3 4
2008 0 0 0 0 0 0 0 0 0 0 3 3
2009 0 0 0 0 0 0 0 0 0 0 3 3
2010 0 0 0 0 0 0 0 0 0 0 2 2
2011 0 0 0 0 0 0 0 0 0 0 2 2
Total 458 1650 2209 2099 1852 1414 920 579 234 76 27 11518
 
Note: Date of birth is missing for three NLSY79 children.

 

Important Information

It is possible to examine cross-sectional patterns of child age at interview by mother's age at birth of child for the entire sample of interviewed children (children ages 0-14 plus young adults) for any survey round. The created variables CINTRVyyyy and YAINTVyyyy can be used to identify all children interviewed in each survey round. Age at interview can be determined using AGECHyyyy for children ages 0-14 and AGEINTyyyy for young adults. Age of mother at birth of child (MAGEBIR) is found in the CHILD BACKGROUND area of interest.

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 (348 of the 6,011 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. This table shows the number of both children and young adults interviewed, by single year of age and by race/ethnicity, in survey years prior to the current round.

Table 3. Number of NLSY79 Child Interviews by Age and Race/Ethnicity: 1986-2014

Sample groups 1986 1988 1990 1992 1994 1996 1998 20003 2002 2004 2006 2008 20104 2012 2014
                               
Total interviews 4,971 6,266 5,801 6,509 7,086 7,102 7,066 6,415 7,466 7,537 7,814 7,658 6,997 6,323 6,011
By age1:                              
   Birth - 14 years         6,107 5,430 4,923 3,390 3,228 2,513 1,970 1,353 895 515 276
   15 years and older          979 1,672 2,1432 3,025 4,238 5,024 5,844 6,305 6,102 5,808 5,735
By race and ethnicity:                              
   Hispanic 937 1,158 1,303 1,483 1,546 1,520 1,550 1,192 1,624 1,648 1,735 1,665 1,521 1,367  1,270
   Black 1,604 1,895 1,994 2,133 2,347 2,329 2,228 1,913 2,412 2,455 2,550 2,521 2,217 2,020 1,922
   Nonblack/non-Hispanic 2,430 3,213 2,504 2,893 3,193 3,253 3,288 3,310 3,430 3,434 3,529 3,472 3,259 2,936  2,819
 
1 Starting in 1994, children who turned age 15 by December 31st of the interview year are interviewed as Young Adults.
2 Young Adults age 21 and older were not fielded in 1998.
3 In 2000, 38% of the black and Hispanic child and young adult (15-20) oversamples were not fielded but were restored to the sample in 2002.
4 Starting in 2010, young adults over age 30 are interviewed only every other round (every four years).

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 980 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 2014. This table shows the number of both children and young adults interviewed by single year of age and by race/ethnicity in earlier rounds.

Table 4. Number of NLSY79 Young Adults Interviewed in 2014 by Age and Race/Ethnicity

Age At Interview Hispanic Black Nonblack/Non-Hispanic Total
 14-16 40 43 107 190
17 22 40 89 151
18 29 37 96 162
19 40 55 92 187
20 44 58 134 236
21 49 86 160 295
22 62 75 151 288
23 64 84 173 321
24 69 93 182 344
25 79 124 190 393
26 77 120 170 367
27 91 110 183 384
28 89 124 162 375
29 89 130 193 412
30 86 125 173 384
      31-33 120 196 175 491
      34-36 91 185 164 440
   37+ 59 164 92 315
Total 1200 1849 2686 5735
     
Note: Starting in 1994, a respondent must have attained the age of 15 by the end of the survey year to be included in the Young Adult survey. Beginning in 2010, respondents over age 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 12/31 of the survey year are not fielded.