1. Department of General Practice, Katholieke Universiteit Leuven, Kapucijnenvoer 33, Bus 7001, 3000 Leuven, Belgium
2. Department of General Practice, Maastricht University, Peter Debyeplein 1, P.O. Box 616, 6200 MD Maastricht, The Netherlands
3. Research Department, NIVEL (Netherlands Institute for Health Services Research), Otterstraat 118–124, P.O. Box 1568, 3500 BN Utrecht, The Netherlands
4. Department of General Practice, EMGO Institute for Health and Care Research, VU University Medical Centre, Van der Boechorststraat 7, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands