[1] Univ Montreal, Sch Rehabil, Montreal, PQ, Canada
[2] Univ Kansas, Shawnee, KS USA
[3] UIowa, Iowa City, IA USA
[4] Boston Univ, Sch Med, Clin Edpidemiol Reserach & Training Unit, Boston, MA 02118 USA
[5] Univ Alabama Birmingham, Birmingham, AL USA
[6] Univ Calif San Francisco, Epidemiol & Biostat, San Francisco, CA 94143 USA