A R Levy1, B Sobolev2. 1. Department of Community Health and Epidemiology, Dalhousie University, #229-5790 University Ave, Halifax, NS, B3H 1V7, Canada. adrian.levy@dal.ca. 2. School of Population and Health, University of British Columbia, 828 West 10th Avenue, Vancouver, BC, V5Z 1M9, Canada.