1. University of Pittsburgh, Graduate School of Public Health, Pittsburgh, PA 15261, USA, Tel.: +1 412 624 7377; Fax: +1 412 624 7397;
2. University of Pittsburgh, Department of Epidemiology, A527 Crabtree, Pittsburgh, PA 15261, USA, Tel.: +1 412 624 3056; Fax: +1 412 624 3737;