The following databases were searched without language restriction from inception to September or November 2006: MEDLINE, EMBASE, CINAHL, DARE, NHS EED, HTA database, the Cochrane Library, Science Citation Index, Web of Science, BIOSIS Previews, National Research Register, Current Controlled Trials, Centre for Reviews and Dissemination (CRD) ongoing reviews and Research Findings Register. Search terms were reported. Only studies reported in English were eligible for inclusion.