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Corresponding author. Hubert Department of Global Health, Rollins School of Public Health, Emory University, 1518 Clifton Road, NE, #722, Atlanta, GA 30322, USA. E-mail: rbsteph@sph.emory.edu
This research was funded by the National Institute of Child Health and Human Development, grant number R01HD039405.
Published by Oxford University Press on behalf of the International Epidemiological Association © The Author 2010; all rights reserved.