[1] Family Hlth Int, Res Triangle Pk, NC 27709 USA
[2] Childrens Hosp Eastern Ontario, Res Inst, Thomas C Chalmers Ctr Systemat Reviews, Ottawa, ON K1H 8L1, Canada
[3] Inst Hlth Sci, Ctr Stat Med, Imperial Canc Res Fund, Med Stat Grp, Oxford, England