The data source has been described in detail elsewhere. In brief, the Nord-Tr0ndelag Health Survey (HUNT)-2 is a large, population-based health survey carried out in the county of Nord-Tr0ndelag in Norway during the years 1995 to 1997. The demographic structure of this county is representative of Norway as a whole, although the level of education and the average income are somewhat lower than the Norwegian averages. All 94,197 county residents > 20 years old were invited to participate in the survey. In all, 62,651 persons (67%) agreed to take part. The participants were asked to fill in extensive questionnaires, including questions on symptoms of reflux and respiratory symptoms. Moreover, data regarding several potentially confounding variables were collected: age, sex, tobacco smoking, body weight and height, alcohol consumption, and use of asthma medications.
Ethical approval for HUNT was obtained from the Regional Committee for Medical Research Ethics, Region IV, Norway. The participants gave extensive written consent to the use of the data. The Norwegian Data Inspectorate has given its approval of the establishment of a research register. Source
Participants of HUNT-2 were asked to report reflux symptoms during the past 12 months together with their severity (“minor” or “major”). The reflux symptoms used in the questionnaire were recurrent heartburn or regurgitation: the cardinal symptoms of reflux. The participants were then classified into three groups based on the reported reflux symptoms. Those without reflux symptoms served as the control group, while those reporting severe symptoms were selected as the case group. The group with minor symptoms was excluded from the study, since the possibility of misclassification of reflux was deemed to be higher in this intermediate group. Such misclassification would decrease the specificity of the reflux disease.