1. Institute of Preventive and Community Dentistry, Dental School, University of Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands
2. Department of Preventive and Community Dentistry, Division of Dentistry, University of Dar es Salaam, P.O.Box 65014, Dar es Salaam, Tanzania
3. Depariment bf Mathematical Statistical Consultation, University of Nijmegen, Toernooiveld 1 Nijmegen, The Netherlands