1. Medical Center Leeuwarden; Department of Pulmonary Diseases; Henri Dunantweg 2 Leeuwarden Netherlands
2. Medical Center Leeuwarden; Department of Epidemiology; Henri Dunantweg 2 Leeuwarden Friesland Netherlands
3. University of Groningen, University Medical Center Groningen; Department of Pulmonary Diseases; Hanzeplein 1 Groningen Netherlands