South Karelia District of Social and Health Services, Lappeenranta, Finland Unit of General Practice, Oulu University Hospital, Oulu, Finland Institute of Health Sciences, University of Oulu, Oulu, Finland Health Centre of Oulu, Oulu, Finland Paediatric Research Centre, Tampere University Hospital and University of Tampere, Tampere, Finland Finnish Institute of Occupational Health, Oulu, Finland Department of Mathematical Sciences, University of Oulu, Oulu, Finland.