1. Institute of Social and Preventive Medicine (ISPM), University of Bern, Finkenhubelweg 11, CH-3012 Bern, Switzerland.
2. Centre for Statistics in Medicine, University of Oxford, Oxford, England.
3. Institute of Social and Preventive Medicine (ISPM), University of Bern, Finkenhubelweg 11, CH-3012 Bern, Switzerland.Department of Social Medicine, University of Bristol, Bristol, England.
4. London School of Hygiene and Tropical Medicine, University of London, London, England.
5. Nordic Cochrane Centre, Copenhagen, Denmark.
6. Department of Clinical Epidemiology, Leiden University Hospital, Leiden, the Netherlands.