Study population
The study was performed within the Obstructive Lung Disease in Northern Sweden (OLIN) Studies. All surveys were approved by the Regional Ethical Review Board at UmeƄ University.
The present paper is based on two random population samples, each followed for ten years. The first sample consists of participants from a postal questionnaire survey of a general population sample in ages 20-69 years, performed in 1996 (n=7104, 85% response rate) [2]. This cohort was followed up with the same questionnaire in 2006, when 5709 individuals participated (80% response rate). The second sample was recruited in 2006 with virtually identical methods (n=6165, 77% response rate) [2], and followed up in 2016 (n=4552, 74% response rate). Individuals from these two cohorts that participated both at baseline and at the follow-up after ten years constitute the study populations in the current paper, n=5709 from cohort recruited in 1996 and n=4552 from the cohort recruited in 2006 (Table 1). The surveys were all performed during the same time period of the year, January to May. We have previously published incidence rates of adult-onset asthma based on a cohort recruited in 1986 in similar ages and in the same area, which was followed-up in 1992 [12] and 1996 [15]. The calculations of incidence in the current paper followed the same protocol as in these previous studies.