1. Department of Clinical Dentistry University of Bergen Bergen Norway
2. Oral Health Center of Expertise in Western Norway Bergen Norway
3. Department of Clinical dentistry University of Bergen Bexbach Germany
4. Department of Preventive Dentistry, College of Dentistry King Faisal University Al Ahsa Saudi Arabia
5. Research Department of Epidemiology and Public Health, WHO Collaborating Centre for Oral Health Inequalities and Public Health University College London London UK