1. a Center for Evaluation Research and Surveys, National Institute of Public Health , Cuernavaca, Morelos, 62440, Mexico
2. b Department of Maternal and Child Health , University of North Carolina at Chapel Hill , North Carolina, USA
3. c Carolina Population Center, University of North Carolina at Chapel Hill , North Carolina, USA
4. d John E. Walker Department of Economics , Clemson University , Clemson, South Carolina, USA
5. e Center for Research in Population Health, National Institute of Public Health , Cuernavaca, México