The effect of multiple reminders on response patterns in a Danish health survey

Background: Reminders are routinely applied in surveys to increase response rates and reduce the possibility of bias. This study examines the effect of multiple reminders on the response rate, non-response bias, prevalence estimates and exposure-outcome relations in a national self-administered health survey. Methods: Data derive from the Danish National Health Survey 2010, in which 298 550 individuals (16 years of age or older) were invited to participate in a cross-sectional survey using a mixed-mode approach (paper and web questionnaires). At least two reminders were sent to non-respondents, and 177 639 individuals completed the questionnaire (59.5%). Response patterns were compared between four groups of individuals (first mailing respondents, second mailing respondents, third mailing respondents and non-respondents). Results: Multiple reminders led to an increase in response rate from 36.7 to 59.5%; however, the inclusion of second and third mailing respondents did not change the overall characteristics of respondents compared with non-respondents. Furthermore, only small changes in prevalence estimates and exposure-outcome relationships were observed when including second and third mailing respondents compared with only first mailing respondents. Conclusions: Multiple reminders were an effective way to increase the response rate in a national Danish health survey. However, when differences do exist between respondents and non-respondents, the results suggest that second and third mailings are unlikely to eliminate these differences. Overall, multiple reminders seemed to have only minor effect on response patterns and study conclusions in the present study.