[1] Stanford Univ, Dept Med, Prevent Res Ctr, Stanford, CA 94305 USA
[2] Stanford Univ, Ctr Hlth Policy, Stanford, CA 94305 USA
[3] Stanford Univ, Ctr Primary Care, Stanford, CA 94305 USA
[4] Stanford Univ, Ctr Outcomes Res, Stanford, CA 94305 USA
[5] Stanford Univ, Ctr Poverty & Inequal, Stanford, CA 94305 USA
[6] London Sch Hyg & Trop Med, Dept Publ Hlth & Policy, London WC1, England
[7] Univ Calif San Francisco, San Francisco Gen Hosp, Ctr Vulnerable Populat, San Francisco, CA 94143 USA
[8] Natl Bur Econ Res, Cambridge, MA 02138 USA