[1] Univ Penn, Div Rheumatol, Ctr Clin Epidemiol & Biostat, Ctr Pharmacoepidemiol & Res Training, Philadelphia, PA 19104 USA
[2] Landspitali Univ Hosp, Div Rheumatol, Reykjavik, Iceland
[3] Univ Penn, Ctr Clin Epidemiol & Biostat, Ctr Pharmacoepidemiol & Res Training, Philadelphia, PA 19104 USA
[4] Boston Univ, Rheumatol Sect, Boston, MA 02215 USA
[5] Boston Univ, Clin Epidemiol Unit, Boston, MA 02215 USA
[6] Univ Penn, Dept Dermatol & Epidemiol, Philadelphia, PA 19104 USA