[1] Acad Med Ctr, Dept Gen Practice, Div Publ Hlth, Amsterdam, Netherlands
[2] Acad Med Ctr, Dutch Cochrane Ctr, Amsterdam, Netherlands
[3] Univ Maastricht, Dept Med Sociol, Maastricht, Netherlands
[4] Grp Hlth Cooperat Puget Sound, Ctr Hlth Studies, MacColl Inst Healthcare Innovat, Seattle, WA 98101 USA
[5] Univ Cambridge, Dept Publ Hlth & Primary Care, Cambridge, England
[6] Free Univ Amsterdam, Med Ctr, Inst Res Extramural Med, Dept Gen Practice, NL-1081 BT Amsterdam, Netherlands