1. Institute of Clinical Medicine, University of Eastern Finland, Kuopio, Finland;
2. Department of Otorhinolaryngology, Turku University Hospital, Turku, Finland;
3. Department of Otorhinolaryngology, Helsinki University Hospital, Helsinki, Finland;
4. Department of Biostatistics, University of Turku, Turku, Finland;
5. Department of Otorhinolaryngology, Kuopio University Hospital, Kuopio, Finland