The Danish health and morbidity surveys: Study design and participant characteristics

Background: Reliable data from health surveys are essential to describe the status and trends in health indicators by means of information not available from official registers. In Denmark, nationally representative health surveys (the Danish Health and Morbidity Surveys) have been carried out among adults during the past three decades by the Danish National Institute of Public Health, University of Southern Denmark. The aim of the present study is to describe the study design of the three most recent surveys in 2010, 2013, and 2017, including the survey mode and response rates. Methods: In 2010, 2013, and 2017, the samples (n = 25,000 each) were based on random sampling of individuals aged 16 years or older with a permanent residence in Denmark. A subsample of previously invited respondents was also re-invited in subsequent survey waves. Data were collected through self-administered questionnaires, yet with a concurrent mixed-mode approach, allowing for the invited individuals to complete either a web questionnaire or an identical paper questionnaire. In 2010 and 2013, survey invitations were sent by regular postal mail, whereas a secure electronical mail service, Digital Post, was used to invite the majority (90.1%) of the sample in 2017. Results: The overall response rate decreased from 60.7% in 2010 to 57.1% in 2013 and 56.1% in 2017. Between 2010 and 2017 the response mode distribution for the web questionnaire increased markedly from 31.7 to 73.8%. The largest increase in the proportion which completed the web questionnaire was found in the oldest age group. Conclusions: Data from the Danish Health and Morbidity Surveys reveal an increasing proportion of the respondents to complete web questionnaires instead of paper questionnaires. Even though the response rate remained relatively stable in 2017, declining response rates is a major concern in health surveys. As the generalizability to the Danish population may be compromised by a low response rate, efforts to increase the response rate or keep it stable are crucial in future surveys. Thus, efforts should be made to ensure convenience and feasibility in relation to access to and the completion of survey (web) questionnaires.