1. O’Neill School of Public and Environmental Affairs, Indiana University , Bloomington, Indiana , USA
2. Department of Epidemiology and Biostatistics, Indiana University School of Public Health , Bloomington, Indiana , USA
3. Department of Epidemiology, University of Michigan School of Public Health , Ann Arbor, Michigan , USA
4. MRC/Wits Rural Public Health and Health Transitions Research Unit (Agincourt), Faculty of Health Sciences, School of Public Health, University of the Witwatersrand , Johannesburg , South Africa