The study design and characteristics of the Danish national health interview surveys

Aims: The Danish National Institute of Public Health, University of Southern Denmark has carried out national representative health interview surveys among adult Danes in 1987, 1994, 2000 and 2005. The aim of this study is to describe the characteristics of the design, including the response rates of the four surveys. Methods: The samples in 1987 and 1994 are based on simple random sampling. The samples in 2000 and 2005 are based on stratified random sampling. In addition, all invited to the survey in 1994 were re-invited in both 2000 and 2005. Data were collected via face-to-face interview at the respondent's home. Following the interview in 1994, 2000 and 2005, all respondents were asked to complete a self-administered questionnaire. Results: The response rate for the face-to-face interview fell from 79.9% in 1987 to 66.7% in 2005 and the response rate for the self-administered questionnaire from 68.1% in 1994 to 51.5% in 2005. The decrease is particularly marked among the young. The mean interview length has increased from 33.3 minutes in 1987 to 50.2 minutes in 2005. Conclusions: The declining response rate in the surveys is a major concern and can pose problems in generalizing data from the surveys to the Danish population. However, these surveys are essential, as the information collected cannot be gathered by means of official statistical registers. Hence, efforts to increase the response rate will be important in the forthcoming surveys.