1. Department of Biostatistics and Bioinformatics; Duke University Medical Center; 2424 Erwin Road, Suite 1102 Hock Plaza Box 2721 Durham NC 27710 U.S.A.
2. Duke Global Health Institute; Duke University; Box 90519 Durham NC 27708 U.S.A.
3. Medical Research Council Biostatistics Unit; Institute of Public Health, University Forvie Site; Robinson Way Cambridge CB2 0SR U.K.
4. International Centre for Eye Health; London School of Hygiene and Tropical Medicine; Keppel Street London WC1E 7HT U.K.