1 Department of Primary Care and Public Health, Leiden University Medical Center, Leiden, the Netherlands. Electronic address: w.p.achterberg@lumc.nl.
2 John Walsh Centre for Rehabiltiation Research, Faculty of Medicine and Health, The University of Sydney, Kolling Institute, Royal North Shore Hospital, New South Wales, Australia.
3 Center for Geriatric Medicine, University of Heidelberg, Germany.
4 Department of Health Services Research and Department of Family Medicine, CAPHRI-Care and Public Health Research Institute, Maastricht University, Maastricht, the Netherlands.