1. Biostatistics, Public Health Department; University of Barcelona; Casanova 143, 08035 Barcelona Spain
2. Center for Research in Environmental Epidemiology (CREAL); Barcelona Spain
3. IMIM (Hospital del Mar Research Institute); Barcelona Spain
4. Centro de Investigacin Biomedica en Red en Epidemiologia y Salud Publica (CIBERESP); Barcelona E-08003 Spain