[1] IWK Hlth Ctr, Ctr Res Family Hlth, Halifax, NS, Canada
[2] Univ Alberta, Dept Pediat, Fac Med & Dent, Edmonton, AB, Canada
[3] McMaster Univ, Dept Clin Epidemiol & Biostat, Hamilton, ON, Canada
[4] McMaster Univ, Offord Ctr Child Studies, Hamilton, ON, Canada
[5] Dalhousie Univ, Dept Community Hlth & Epidemiol, Halifax, NS, Canada
[6] IWK Hlth Ctr, 5850-5980 Univ Ave, Halifax, NS, Canada
[7] Univ Alberta, Dept Pediat, 3-526 Edmonton Clin Hlth Acad,11405-87 Ave, Edmonton, AB T6G 1C9, Canada