The impact of reminders on representativeness and survey estimates among web-mode invited in the Danish National Health Survey

Declining response proportions in health surveys may lead to increased non-response bias. Multiple reminders are often used to increase response proportions, and, thus, we aimed to determine if the use of reminders decreased the magnitude of non-response bias among web-mode invited in the Danish National Health Survey 2023. In the Danish National Health Survey 2023, a national random sample of 23 467 individuals (aged ≥16 years) with residence in Denmark were invited by a secure electronic mail service. Invited individuals received up to five inquiries: (i) web invitation, (ii) web reminder, (iii) paper invitation including a questionnaire and a prepaid return envelope, (iv) paper reminder, and (v) paper reminder including a questionnaire and a prepaid return envelope. The cumulative response proportions after first-, second-, third-, fourth-, and fifth inquiries were 19.1%, 28.9%, 37.2%, 39.3%, and 42.0%, respectively. In general, third, fourth, and fifth mailing respondents were more often men, at younger ages, with non-Western backgrounds, and unmarried compared to first mailing respondents. Furthermore, third, fourth, and fifth mailing respondents were in general found to have less favorable health behavior than first mailing respondents, but also a lower prevalence of fair or poor self-rated health and long-standing health problems. In conlusion, reminders are an effective way to increase the response proportion. Furthermore, the use of reminders was found to decrease the magnitude of non-response bias; however, the decrease was small due to the low number of individuals responding after fourth and fifth inquiries.