1. UCLA Department of Medicine , Los Angeles, California, USA
2. Department of Epidemiology, University of Iowa , Iowa City, Iowa, USA
3. Palmer Center for Chiropractic Research, Palmer College of Chiropractic , Davenport, Iowa, USA
4. RAND Health Care, RAND Corporation , Santa Monica, California, USA
5. Parker University , Dallas, Texas, USA
6. Samueli Foundation, Corona del Mar , California, USA
7. Department of Orthopaedic Surgery, Duke University , Durham, North Carolina, USA