[1] Univ Calif San Diego, Joint Doctoral Program Epidemiol, San Diego State Univ, San Diego, CA 92109 USA
[2] Univ Calif San Diego, Dept Family & Prevent Med, Sch Med, San Diego, CA 92109 USA
[3] Kaiser Permanente, Surg Outcomes & Anal Dept, San Diego, CA USA
[4] So Calif Permanente Med Grp, Dept Orthopaed Surg, El Cajon, CA USA