The interview methodology of the SOEP is based on a set of pre-tested questionnaires for households and individuals. Interviewers try to obtain face-to-face interviews with all members aged 16 years and over of a given survey household. Thus, there are no proxy interviews for adult household members. Additionally, one person (the so called “head of household”) is asked to answer a household related questionnaire covering information on housing, housing costs, and different sources of income (e.g. social transfers like social assistance or housing allowances). This questionnaire also covers some questions on children in the household up to the age of 16, mainly concerning their attendance in day care, kindergarten and school.
The questions in the SOEP are in principle identical for all particpants of the survey to ensure comparability across the participants within any given year (of course, there are differences across years. There are a few exceptions to this rule, which are due to different requirements in the target population. Up to 1996 the questionnaires for the foreigner’s sample (B) and immigrant sample (D) covered additional measures of integration or information on re-migration behavior. Between 1990 and 1992, i.e. during the first years of the German unification process, the questionnaire for the East German sample (C) also contained some additional specific variables. Since 1996, all questionnaires are uniform and completely integrated for all main SOEP samples. The related studies use SOEP related content, but also have specific questions, so the contents may differ to various degrees in every year.
Another type of difference in questionnaires is implemented because first time respondents are not treated identically to those with a repeated interview, since some information does not have to be asked every year unless a change occurred. Additionally, each respondent is asked to fill out a biography questionnaire covering information on the life course up to the first SOEP interview (e.g. marital history, social background, and employment biography).
Additional information - not provided directly by the respondents - can be obtained from the so-called “address logs”, which are stored for every year in the $PBRUTTO and $HBRUTTO files. Every address log is filled in by the interviewer even in the case of non-response, thus providing very valuable information, e.g. for attrition analyses. For researchers interested in methodological issues these data also contain information on the field work process, e.g. the number of contacts, reason for eventual drop-outs, or the interview mode. For successfully contacted households, the address logs cover the size of the household, some regional information, survey status etc., while the individual data for all household members include the relation to the household head, survey status of the individual and some demographic information.
Measuring stability and detecting changes means to repeat (almost) identical measures over time. Furthermore, the SOEP-questions capture stability and change by varying with regard to the time dimension, asking about events in the past, the present, and the future. Conceptually, different measurements of time are used:
The SOEP uses several different modes to collect the data. Originally, the respondent’s answers were recorded by an interviewer who filled in a paper questionnaire, the so called pen-and-paper interview or PAPI. The personal contact between interviewer and respondent is important for the success of the survey; however, before losing a respondent because of a scheduling conflict between interviewer and respondent, the SOEP allows mailing in the questionnaire starting from the second wave of subsamples A-I. This concept does not resemble the concept of a regular mail survey, because the interviewer still keeps the personal contact with the household and schedules appointments with its respondents if possible. Starting with subsample J, only the computer assisted mode (CAPI) is allowed, and thus mailing in the questionnaires is no longer possible.
While the interviewer is in the household she/he directly conducts an interview with any household member, but can also hand out a questionnaire to other household members, who fill it in with or without her/his help (self-administered questionnaires, SAQ). This is much more time efficient for the interviewer, because household members can work in parallel on their questionnaires.
In 1998, interviews were conducted with computers for the first time, in computer-assisted personal interviews, or in CAPI mode. Compared to PAPI, CAPI is much more efficient in transferring the data into an electronic format, which was an important asset especially with the extensions of the panel starting in the year 2000. The CAPI mode was first conducted in parallel to the PAPI mode, meaning that interviewers and respondents were free to chose how they wanted to do the interview. This was important for the “older” sample members (responents as well as interviewers), who were used to the PAPI concept. Only in the most recent samples (starting in subsample J), CAPI is the only mode. Figure 5 depicts the development of modes up to 2011, showing that the CAPI mode has gained importance since its implementation.
Since the questionnaires have to be identical in both modes, the CAPI implementation is relatively simple compared to what would be technically feasible. For example, the SOEP basically does not use any form of dependent interviewing (i.e. referring to respondent data from previous waves), because this cannot be easily implemented in the PAPI-mode. Also, the filtering structure is very simple in the SOEP, because any respondent must be able to follow the interview path on her/his own on paper. Still, some technical features like the control of value ranges (e.g. month of birth, year of first marriage) or the randomization of scale items are implemented in the CAPI version of the questionnaire.
In the future, new modes will be introduced into the SOEP as they develop. The computer-assisted web interview (CAWI) is close to implementation, it will, however, not be used as a replacement of the current CAPI and PAPI modes, but rather as an extension the respondents may use similar to the mail-in or self-administered questionnaires. The core interview concept of the SOEP survey, the personal contact between respondent and interviewer, will not change.
Figure 5: Use of Different Interviewe Modes since 1984
To cope with panel attrition and to keep the the longitudinal response rates at high levels, the SOEP has implemented so-called “panel care” efforts to maintain the personal contact between respondents and the survey. Panel care can be divided into incentives directly given to the respondent and other measures undertaken to keep the respondent in the study.
The study has honored the respondents with gifts and tokens of appreciation from the very beginning. For the most part, these gifts are small in-kind incentives like flowers, for which the interviewers have their own budget. In addition, the interviewers are asked to hand out a brochure with recent results from the study. Up to 2007, the respondents also received a lottery ticket as a thank you upon completion of the interview. The lottery collects money for social projects in Germany. Since 2008, the lottery ticket is included in the contact letter which is sent out about two weeks prior to the interview. It is thus given unconditionally, as long as the person has participated in the previous wave. After any successful interview, the respondent receives a thank you letter from the field work organisation, which also inlcudes a stamp for a regular letter.
In 2009, different incentive schemes were tested in the new subsample I to increase the first-wave response rates. The basic experiment included four randomized groups of households: (1) those with the default setup of the conditional lottery ticket; (2) those with a “low” cash incentive involing 5 Euros per household and 5 Euros per adult respondent; (3) those with a “high” cash incentive involing 5 Euros per household and 10 Euros per adult respondent; and (4) those with a choice between a “low” cash incentive and a lottery ticket. The results showed slightly higher response rates in the cash groups, although the extra money in group (3) did not pay of. (Further results will be published on our website as soon as possible.) Additional work is done by the field work agency: Addresses are kept up to date throughout the year in order to be informed about residential mobility. This is achieved for example by sending out a brochure containing some results based on previously collected data, or saisonal greeting cards.
In addition, the face-to-face interview ensures a personal relationship, which increase the likelihood to stay in the survey. Thus, keeping the same interviewer over time is one important goal - some of the respondents have indeed had the same interviewer since the beginning in 1984.