1. African Population and Health Research Center, Population Dynamics and Urbanization Theme, Nairobi, Kenya;
2. The University of Iowa, Department of Biostatistics, Iowa City, IA, USA;
3. The University of North Carolina‚ School of Medicine, Center for Health Equity Research, Chapel Hill, NC, USA;
4. The University of Iowa College of Public Health, Department of Occupational and Environmental Health, Iowa City, IA, USA