[1] Ann Arbor Vet Affairs Med Ctr, Ann Arbor, MI USA
[2] Univ Michigan, Dept Internal Med, Div Gastroenterol, Med Sch, Ann Arbor, MI 48109 USA
[3] Univ Michigan, Dept Epidemiol, Sch Publ Hlth, Ann Arbor, MI 48109 USA
[4] Univ Michigan, Inst Social Res, Ann Arbor, MI USA
[5] Univ Michigan, Sch Publ Hlth, Dept Biostat, Ann Arbor, MI 48109 USA