1. School of Public Health of Mexico at the National Institute of Public Health of Mexico, Av. Universidad 655, Santa María Ahuacatitlán, 62100 Cuernavaca, Morelos, Mexico
2. Center for Health System Research, National Institute of Public Health of Mexico, Av. Universidad 655, Santa María Ahuacatitlán, 62100 Cuernavaca, Morelos, Mexico
3. School of Economics, National Autonomous University of Mexico, Interior S/N, C.U., Coyoacán, 04510 Mexico City, Mexico
4. National School of Nursing and Obstetrics, National Autonomous University of Mexico, Camino Viejo to Xochimilco and, Viad. Tlalpan, Huipulco, 14370 Mexico City, Mexico