To cope with panel attrition and to keep 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 organization, which also includes 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 involving 5 Euros per household and 5 Euros per adult respondent; (3) those with a “high” cash incentive involving 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. 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 seasonal 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.