[1] Stanford Univ, Med Ctr, Div Gastroenterol & Hepatol, Sch Med, Stanford, CA 94305 USA
[2] Stanford Univ, Dept Stat, Stanford, CA 94305 USA
[3] VA Palo Alto Hlth Care Syst, GRECC, Palo Alto, CA USA
[4] Stanford Univ, Sch Med, Dept Med, Ctr Primary Care & Outcomes Res, Stanford, CA 94305 USA