[1] Canadian Coll Naturopath Med, Dept Res & Clin Epidemiol, Toronto, ON M2K 1E2, Canada
[2] London Sch Hyg & Trop Med, London WC1, England
[3] McMaster Univ, Hamilton, ON, Canada
[4] Ottawa Hlth Res Inst, Clin Epidemiol Program, Ottawa Methods Ctr, Ottawa, ON, Canada