Created Variables

CC#DOB: Dates of birth of each child
C#SEX: Gender of each child
C#ID: Identification number of each child
C#RES_DLI: Usual residence of each child at most recent interview
C#DOD: Date of death of each child
NUMKID: Number of children ever born to the respondent as of the latest interview
AGE#B: Age at first, second and third birth
MO#B#B: Months between first/second and second/third births
AGE#M: Age began first marriage
MOBG#M/YRBG#M: Month/Year began first/second/third marriage
MOEN#M/YREN#M: Month/Year ended first/second marriage
MO1M1B: Absolute value of months between first marriage and first birth
FL1M1B: A flag indicating whether first marriage occurred before first birth
MOBG1P/YRBG1P: Month/year that first pregnancy began
OUT1P: Outcome of the first pregnancy
AGE1P: Age of respondent at start of the first pregnancy
PREGS, MISCAR, ABORTS: Number of pregnancies, miscarriages/stillbirths and abortions respectively


Important Information About Using Fertility Data

Researchers constructing pregnancy histories should understand a subtle change that began with the 1992 survey.  Prior to 1992, the questionnaire asked female respondents to report about pregnancy episodes since the last fertility questions were asked (usually two years earlier).  Beginning in 1992, the questionnaire asked respondents detailed questions about pregnancies which ended in a live birth only.  While the total number of pregnancies can be determined, distinctions between miscarriages and stillbirths are not made.  In addition, while dates of all abortions are collected through the confidential card, only the end date is collected in the fertility section of the first pregnancy that did not result in a live birth.  The specific outcome of this pregnancy was not asked and thus could be a stillbirth, miscarriage, or abortion. The "Fertility and Relationship History/Created" area of interest has been part of the NLSY79 data set for many years and consists of edited and constructed variables that incorporate the results of a significant cleaning and editing process begun in the early 1980s.  This effort began as part of an evaluation of the retrospective fertility data collection in 1982/1983 in comparison to base year and updated collections that took place in 1979, 1980, and 1981.  Additional evaluations have been conducted periodically since then.  For further information on the quality of NLSY79 fertility data, users should see Mott et al. (1983) and Mott (1985, 1998). A complete description of the contents of the "Fertility and Relationship History/Created" area of interest is provided in Appendix 5 of the NLSY79 Codebook Supplement and also describes how the data were checked, lists research reports that investigate the quality of the data, and explains special coding and edit flags.

Every round of the NLSY79 has included a section on fertility. The first three rounds of the NLSY79 (1979, 1980, and 1981) have very short fertility sections. 

Table 1. Year-by-Year Variations in Fertility Data Collection

Variable 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94-2012
Birthdates of any children born (early years since last interview; updates on children at last interview; additional children born since last interview) * * * * * * * * * * * * * * * *
Sex, usual residence, death date (if applicable) for children (updates on children from last interview; children born since last interview)        *  *  *  *  *  *  *  *  *  *  *  *  *
Contraception/wantedness (females only)       * * * * *   *   *   *   *
Pregnancy history (females only)       * * * *                  
Pre/post/neonatal care (females only)         * * *   *   *   *   *
Abortions           * * *   *   *    *    *
Sex education            *                    
Left school due to pregnancy (females only)           *                    
Visitation information           * * *   *   *   *   *
Total number children desired *      *  *  *  *    *    *    *    *
Total number children expected *                              
NON-BIOLOGICAL CHILDREN                                
Birthdates, gender              *  *    *    *      *
Adopted or step child                        *    *    *

Fertility History

Researchers can create fertility event histories in a number of ways.  One method is to extract the variables from each year's survey data which record when each child was born.  However, NLSY79 data show that this is not an accurate method for creating an event history.  In each survey, respondents are asked to correct information in the fertility roster.  (Prior to 1993, this was the Children's Record Form or CRF.  Beginning in 1993, this is the BIO/NBIO Child Roster).  Each year, numerous changes are made.  For example, in the 1994 survey, parents changed some portion of the birth record for 548 children.  While this number appears high, the vast majority of changes are to the spelling of children's names.

Because the raw recorded data on dates of birth, sex, and status (adopted, deceased) entered on the Children's Record Form (CRF) are subject to interviewer as well as respondent error, NLSY79 staff created a fertility event history beginning in 1982.  This was done to aid users and, at the same time, evaluate the quality of the NLSY79 fertility data.  This series, found in the "Fertility and Relationship History/Created" area of interest, lists a number of variables including the birth month, and year of every child born to NLSY79 respondents; numbers and outcomes of pregnancies; ages of respondents at the birth of selected children; dates of death for children who have died; and usual residence of children.

Age at First Birth

Beginning in 1982, every NLSY79 data release has included a created variable that tracks the age of respondents when they first give birth.  Readers can also find created variables in the "Fertility and Relationship History" area of interest that track how old the respondent was when the second and third births occurred.

Fertility Expectations and Desires

The fertility section provides researchers with an opportunity to examine whether expectations about the future compare accurately with what actually happens in respondents' lives.  In 1979 and 1982, respondents were asked how many children they desired. 

In 1979, 1982, 1983, 1984, 1985, 1986, and every even year since 1986, respondents have also reported on the number of children they actually expect to have. 

Fertility and Relationship History

The 2000 release of the NLSY79 data introduced a new series of constructed variables for each survey year that provide information about the respondent's relationship status.  All surveys were examined to match names of spouses or partners for the entire administration of the NLSY79.  The first variable provides information about the respondent's total number of spouses or partners since 1979 and can range from 0 to 9.  The percentage of respondents who have never reported having a spouse or partner can be found in Table 2.

Second, a unique code is assigned to each new spouse/partner at the interview date.  If the same spouse or partner resides with the respondent during the next survey round, the spouse or partner code remains the same.  If the respondent has a new spouse or partner, the next available number is given to that person.  If in a later survey round the respondent is reunited with a previous spouse or partner, the number of spouse/partners is not increased, and the code for that year reflects that spouse/partner's original number.

Table 2. Percentage of Respondents Who Have Never Reported Having a Spouse or Partner

Year Percentage Year Percentage
1979 88.1 1992 20.0
1980 82.0 1993 18.5
1981 74.7 1994 17.1
1982 66.8 1996 15.3
1983 58.8 1998 13.8
1984 52.1 2000 12.6
1985 47.8 2002 11.9
1986 41.1 2004 11.0
1987 35.3 2005 10.9
1988 29.8 2008 10.4
1989 25.8 2010 10.2
1990 22.9 2012 9.94
1991 22.0    

The second created variable reveals whether the person living with the respondent is a spouse, coded 1, or a partner, coded 33.  In some instances, the exact relationship is indeterminate in a given round; in these cases, a code of 36 is used. If no spouse or partner is present at a given survey point, but the respondent has reported a spouse/partner in the past, their relationship code will be zero.  Respondents with no known spouses/partners receive a code of -999. 

There is no guarantee that the same individual(s) were present between survey rounds or that all partners are accounted for, as a spouse or partner may have appeared between survey rounds but not have been present at any survey point.  Early examination suggests that this applies to only a modest proportion of cases. In some instances, identification of spouses who only were present between rounds may be possible by using the NLSY79 marriage history, as well as the marriage transition information available at each survey point. Survey Instruments and Documentation:  Information is found in the "Fertility" section of each questionnaire.  For additional details on the edited and cleaned fertility data found in area of interest "Fertility and Relationship History/Created," see Appendix 5 in the NLSY79 Codebook Supplement.

Comparison to Other NLS Cohorts: Maternal prenatal care information and health-related characteristics are provided on the NLSY79 Child and Young Adult file and are linked to individual children.

Information on fertility and on the status of children has been regularly collected from the NLSY97 and the Original Cohorts.  Data include number of children, dates of birth, sex, and life status. For more precise details about the content of each survey, consult the appropriate cohort's User's Guide using the tabs above for more information.


Mott, Frank L., Paula Baker, R. Jean Haurin, and William Marsiglio, "Fertility Related Data in the 1982 National Longitudinal Survey of Work Experience of Youth:  An Evaluation of Data Quality and Preliminary Analytical Results." Columbus, OH:  CHRR, The Ohio State University, 1983.

Mott, Frank L. "Evaluation of Fertility Data and Preliminary Analytical Results from the 1983 (Round 5) Survey of the National Longitudinal Surveys of Work Experience of Youth." Columbus, OH:  CHRR, The Ohio State University, 1985.

Mott, Frank L. "Male Data Collection:  Inferences from the National Longitudinal Surveys." Columbus, OH:  CHRR, The Ohio State University, 1998.

Survey Instruments and Documentation See the "Sexual Activity & Contraception" and "Child Care" sections of this guide.  For information on nonbiological children, see the "Household Composition" or "Gender" sections of this guide and areas of interest "Child Record Form/Nonbiological" and "Birth Record xxxx."
Areas of Interest Raw unedited data on fertility, pregnancy, sexual activity, and contraception can be found in areas of interest "Children," "Child Record Form/Nonbiological," "Birth Record," and "Birth Record xxxx."  Area of interest "Fertility and Relationship History/Created" contains edited and created variables on fertility, pregnancy, and marriage.