- Correspondence to Dr Marcel Zwahlen, Institute of Social and Preventive Medicine, University of Bern, Finkenhubelweg 11, Bern CH 3012, Switzerland;
- Received 12 November 2017
- Revised 5 December 2017
- Accepted 7 December 2017
- Published in print 30 December 2017.
- Published online 29 December 2017.
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