Interview Methods

Interview Methods

Interview Methods and Target Universe

During each survey round, NORC attempts to reach all youth within the active samples. No respondents have been routinely excluded from locating efforts with the exception of respondents who have died or, in a small number of cases, were judged to be extremely difficult. The permanent NLSY79 sample designated for interviewing during the 1979-84 interview years consisted of all civilian and military youth who were interviewed in the base year and who were alive at the survey date.

In 1985, when interviewing of the full military sample ceased, the total NLSY79 sample size dropped from 12,686 to 11,607. Retained for interviewing in post-1984 surveys were 201 military respondents randomly selected from the entire military sample of 1,280; the remaining 1,079 military respondents were eliminated from the sample. The 201 military members who were retained included (1) 51 cases that would have been selected as part of a random sample of youth including the military and (2) 150 additional cases selected to provide a sufficient number of original military sample members to avoid overly large sampling variability for the military sample. Beginning in 1991, the 1,643 members of the economically disadvantaged, nonblack/non-Hispanic supplemental sample were no longer interviewed. Unless otherwise noted, eligible sample sizes reported in NLS publications include deceased and difficult-to-field respondents but exclude those respondents dropped from the sample. Additional information on numbers and characteristics of noninterviewed respondents can be found in the Reasons for Noninterview section.

NLSY79 respondents reside in each of the 50 States and the District of Columbia, U.S. territories, and other countries. Prior to fielding, respondents receive a short, informative advance letter reminding him or her of the upcoming interview and confirming the respondent's current address and phone number. As of the 2006 round, all main youth respondents are instructed to call and set up an appointment for their interview.

Field staff locating efforts begin when there is no contact from respondents. Using locator sheets, interviewers at the local level are responsible for contacting all respondents in their caseloads and for tapping additional local resources (post offices, departments of motor vehicles and vital statistics, and so forth) to locate those respondents who have moved. If an interviewer is unsuccessful in locating a respondent, the case is transferred to the field manager who undertakes additional locating strategies.

In the event that such local-level efforts fail, the case is forwarded to NORC's locating shop where the complete files on each respondent can be accessed and used for additional locating efforts. Respondents who cannot be located are only a small percentage of the total not interviewed in a given survey year. (For more information about noninterviews, refer to the Reasons for Noninterview section.)

In addition to its comprehensive locating efforts, NORC makes every effort to convert initial respondent refusals to completed interviews. Uncooperative respondents receive "refusal conversion letters" and a wide array of materials designed to encourage continued participation in the survey. Extensive locating methods and a strong conversion strategy, combined with close monitoring of response rates for each of the subsamples of the NLSY79, have resulted in relatively high retention rates for a longitudinal panel of this duration. 

In early rounds, telephone contact to complete the survey occurred under certain circumstances, such as 1987 when funding restrictions limited in-person interviews, or in any year, when the respondent resided in a remote area or field staff determined that phone contact was the preferred method of interviewing a respondent. Through the years, respondents have become more dispersed or expressed a preference for phone interviews. In response the number of telephone interviews increased greatly beginning in 2002 and is now the main mode of interviewing. The percent of surveys conducted by telephone for each survey year are shown in Table 1.

Table 1. Percent of NLSY79 Interviews Conducted by Telephone, 1979-2020

Year Number of Phone Interviews Percent of Total Interviews   Year Number of Phone Interviews Percent of Total Interviews
1979 548 4.4  19931 - -
1980 648 5.3  19941 - -
1981 654 5.4 1996 1042 12.1
1982 1054 8.7 1998 2069 24.6
1983 324 2.6 2000 2613 32.5
1984 646 5.3 2002 5407 70.0
1985 953 8.7 2004 6497 82.8
1986 929 8.7 2006 6542 85.5
1987 8998 85.8 2008 6875 88.8
1988 920 8.8 2010 6799 90.1
1989 1518 14.3 2012 6697 91.8
1990 1317 12.6 2014 6861 97.1
1991 1241 13.8 2016 6646 96.3
1992 1164 12.9  2018  6592  95.8 
        2020 6407 98.0
1 Questions identifying whether interview was conducted by telephone were not included in the 1993 and 1994 surveys.

In rare cases, interviews are conducted in whole or in part with a proxy, a person other than the respondent (for example, four in 1991, two in 1992). A variable, entitled 'Interview Conducted with Proxy Respondent,' is present in the data to identify these interviews. In order to conduct such an interview, individual approval must be obtained by the NORC central office and the circumstances documented.

A Spanish version of the NLSY79 is prepared and NORC employs bilingual, Spanish-speaking interviewers. During the 2014 interview, for example, 93 respondents requested a Spanish version of the questionnaire.

The average length of a personal interview is approximately one hour. The 1987 telephone interviews were completed within about 40 minutes, while the administration of the child assessments added approximately 45 minutes to the total survey administration time for each child. 

Until 1989, the NLSY79 was conducted using only paper-and-pencil interviews (PAPI). PAPI interviews were performed by interviewers filling in the relevant fields of large printed questionnaire booklets. While these booklets were inexpensive to produce, interviewers could make mistakes in following complicated skip patterns and filling in answers. Moreover, after all interviews were completed, additional office staff were needed to transcribe the information collected. Computer-assisted personal interviews (CAPI) were designed to eliminate many of these problems. With CAPI, interviewers take laptop computers into the field instead of questionnaire booklets. A computer program automatically selects the next question, prevents interviewers from entering illegal values, and warns interviewers about questionable answers. The computer also eliminates the need for data transcription except for specific items collected verbally and coded later.

While the majority of interviews in 1990 were collected using PAPI materials, a subset of one fourth of respondents was administered their survey using CAPI methods in order to test the viability and reliability of CAPI administration. Due to the success of these experiments, the NLSY79 interviews became fully CAPI administered beginning in 1993. Users interested in the results of these experiments should consult Olsen (1991).  

Interview Schedule and Fielding Periods

The original interview schedule, which called for yearly personal interviews with NLSY79 respondents, was retained from 1979 through 1986. In 1987, budget constraints dictated a limited phone interview rather than a personal interview. Personal interviews resumed with the 1988 round and continued yearly until 1994.  NLSY79 respondents have been interviewed in even-numbered years since 1994.

The initial NLSY79 interviews were conducted between late January and mid-August 1979. The next several interviews were fielded in the first six months of the year; more recently surveys have typically begun in winter and ended the following winter. Table 2 provides information on the fielding periods for NLSY79 respondents.

Note: There is no timing requirement that interviews take place on the same day/month from round to round. Thus, a respondent interviewed at the beginning of one field period and then interviewed at the latter end of the next field period would have more weeks between interviews than someone who was interviewed the same time both survey years. Also, some respondents, for various reasons, may miss multiple survey rounds before they are interviewed again. This means that the number of weeks since respondents' last interview can vary greatly. The LINT-DATE variables give the date last interviewed.

Table 2. NLSY79 Fielding Periods

Survey Year(s) Fielding Period
1979-80 January-August
1981-82 January-July
1983-85 January-June
1986 February-July
1987 March-October
1988-91 June-December
1992 May-December
1993 June-November
1994 June-December
1996 April-October
1998 March-September
2000 April 2000-January 2001
2002 January-December
2004 January 2004-February 2005
2006 January 2006-March 2007
2008 January 2008-March 2009
2010 December 2009-February 2011
2012 September 2012-September 2013
2014 October 2014-October 2015
2016 October 2016-November 2017
2018  September 2018 - November 2019
2020 September 2020 - December 2021

From 1979 until 1986, timing of the fielding period was designed to allow all respondents still in school to be interviewed before they left to take temporary summer jobs. Detailed information was collected for jobs held by respondents while they were in school. Since the youngest respondents in the survey were 23 years old in 1988, the shift in fielding periods after 1987 had a relatively small impact on information on jobs held while in school. An attempt was made during the initial survey years to keep the fielding period for an individual respondent approximately the same from year to year in order to assure that the time between interviews was approximately twelve months.

Researchers conducting analyses on topics where time periods are critical should carefully examine the reference period of the questions, the actual interview date, and the duration since the preceding interview.