1. Dept. of Hygiene and Epidemiology, Athens University Medical School, Athens 11527, Greece, Correspondence address: E. Petridou Center for Research of Prevention of Injuries among the Young Dept. of Hygiene and Epidemiology Athens University Medical School 75 M. Asias Str., GR-11527 Athens, Greece Tel: 301 7773840 Fax: 301 7773840 or 7704225
2. 2nd Dept. of Pediatrics, A. Kyriacou Children's Hospital, Athens, Greece
3. Dept. of Hygiene and Epidemiology, Athens University Medical School, Athens 11527, Greece
4. Dept. of Epidemiology, Harvard School of Public Health, Boston, MA 02115, USA