To Remind or Not to Remind During Recruitment? An Analysis of an Online Panel in Germany.

Objective: To explore the role of reminders in recruiting and maintaining participation in an online panel. Methods: 50,045 individuals from five German federal states were invited by regular mail to participate in the online study "Health-Related Beliefs and Healthcare Experiences in Germany." Those who did not respond to the first attempt received a postal reminder. Comparisons of sociodemographic characteristics and responses were made between first-attempt respondents and those who enrolled after the second letter. Results: After the initial letter, 2,216 (4.4%, 95%CI: 4.3%-4.6%) registered for the study; after a reminder 1,130 (2.5%, 2.3%-2.6% of those reminded) enrolled. Minor sociodemographic differences were observed between the groups and the content of the responses did not differ. Second-attempt respondents were less likely to participate in subsequent questionnaires: 67.3% of first-attempt vs. 43.3% of second-attempt respondents participated in their fourth survey. Recruitment costs were 79% higher for second-attempt respondents. Conclusion: While reminders increased the number of participants, lower cost-effectiveness and higher attrition of second-attempt respondents support the use of single invitation only for studies with a similar design to ours when the overall participation is low.