1. Department of Epidemiology, Mailman School of Public Health, Columbia University, 722 West 168th Street Room 809, New York, NY 10032, USA
2. Division of Physical Therapy, Department of Allied Health Sciences, School of Medicine and The Cecil G. Sheps Center for Health Services Research, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
3. Department of Health Behavior and Health Education, Gillings School of Global Public Health and the Thurston Arthritis Research Center, School of Medicine, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
4. Department of Physical Therapy, University of Alberta, Edmonton, AB, Canada T6G 2R3
5. Department of Medicine, School of Medicine and the Cecil G. Sheps Center for Health Services Research, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA