[1] Univ Pittsburgh, Sch Med, Dept Med, Pittsburgh, PA 15213 USA
[2] Univ Pittsburgh, Sch Med, Dept Biomed Informat, Pittsburgh, PA 15213 USA
[3] RAND Corp, Pittsburgh, PA USA
[4] RAND Corp, Santa Monica, CA USA
[5] Univ N Carolina, Sch Med, Div Gastroenterol, Ctr Gastrointestinal Biol & Dis, Chapel Hill, NC USA
[6] Northwestern Univ, Dept Med, Feinberg Sch Med, Chicago, IL 60611 USA