Sample Design

Sample Design

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. 

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

 Birth Year
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 354 703 166 0 0 0 0 0 0 0 0 1223
1979 55 206 267 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 0 164 294 245 0 0 0 0 0 0 0 703
1983 0 106 262 303 44 0 0 0 0 0 0 715
1984 0 32 233 270 122 0 0 0 0 0 0 657
1985 0 0 226 256 194 0 0 0 0 0 0 676
1986 0 0 100 237 231 49 0 0 0 0 0 617
1987 0 0 23 247 251 100 0 0 0 0 0 621
1988 0 0 0 206 224 154 0 0 0 0 0 584
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 0 175 164 84 0 0 0 0 423
1992 0 0 0 0 84 195 109 14 0 0 0 402
1993 0 0 0 0 29 170 111 38 0 0 0 348
1994 0 0 0 0 0 115 134 54 0 0 0 303
1995 0 0 0 0 0 58 115 64 7 0 0 244
1996 0 0 0 0 0 25 125 66 13 0 0 229
1997 0 0 0 0 0 0 105 90 20 0 0 215
1998 0 0 0 0 0 0 35 85 35 1 0 156
1999 0 0 0 0 0 0 14 86 23 6 0 129
2000 0 0 0 0 0 0 0 54 35 10 0 99
2001 0 0 0 0 0 0 0 30 38 13 0 81
2002 0 0 0 0 0 0 0 2 33 11 2 48
2003 0 0 0 0 0 0 0 0 20 14 0 34
2004 0 0 0 0 0 0 0 0 12 9 3 24
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
2014 0 0 0 0 0 0 0 0 0 0 1 1
                         
                         
Total 459 1652 2209 2097 1855 1417 927 583 239 79 28 11545
 
Note: Date of birth is incomplete for 6 NLSY79 children.

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. 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: 1986-2020

Sample groups 86 88 90 92 94 96 98 003 02 04 06 08 104 12 14 165 185 205
                                     
Total interviews 4971 6266 5801 6509 7086 7102 7066 6415 7466 7537 7814 7658 6997 6323 6011  5391 4965 4354
By sample type                                    
   Child         6107 5430 4923 3390 3228 2513 1970 1353 895 515 276  39    
   Young Adult          979 1672 21432 3025 4238 5024 5844 6305 6102 5808 5735  5352 4965 4354
By race and ethnicity:                                    
   Hispanic 937 1158 1303 1483 1546 1520 1550 1192 1624 1648 1735 1665 1521 1367  1270 1156 1041 939
   Black 1604 1895 1994 2133 2347 2329 2228 1913 2412 2455 2550 2521 2217 2020 1922 1754 1555 1384
   Nonblack/ non-Hispanic 2430 3213 2504 2893 3193 3253 3288 3310 3430 3434 3529 3472 3259 2936  2819  2481 2369 2031
 
1From 1994 to 2014, children age 15 and older by the end of the survey year were interviewed as Young Adults. Beginning in 2016, the Young Adult has included children 12 and older by the end of the survey year.
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).
5Starting in 2016, children 12 to 14 were included in the Young Adult sample.

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.

Table 4. Number of NLSY79 Young Adults Interviewed in 2020 by Age and Race/Ethnicity

Age At Interview Hispanic Black Nonblack/
Non-Hispanic
Total
12-13 0 2 4 6
 14-16 11 9 19 39
17 11 4 17 32
18 7 9 20 36
19 13 14 28 55
20 14 28 37 79
21 25 21 52 98
22 21 22 76 119
23 30 37 97 164
24 26 39 94 159
25 40 48 101 189
26 43 61 119 223
27 51 68 137 256
28 59 73 149 281
29 56 81 163 300
30 55 82 145 282
      31-33 87 110 177 374
      34-36 93 120 175 388
     37-39 178 266 264 708
  40+ 119 209 157 566
Total 939 1384 2031 4354
     
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 have not been  fielded. Beginning in 2012, children ages 12 to 14 were included in the Young Adult sample.