Methods
The search strategy identified 117 studies, of which 29 were eligible for inclusion and 8 met the inclusion criteria. All the studies meeting the inclusion criteria (Brown 2002; Cloud 2004; Davis 2002; Kahler 2004;MATCH 1998; McCrady 1996; Walsh 1991; Zemore 2004) were randomised controlled studies. Twenty two studies were excluded and the reasons are explained in the table ′Characteristics of excluded studies′. Briefly these reasons can be summarized as: study design not in the inclusion criteria (ten studies); objectives not in the inclusion criteria (three studies); intervention not in the inclusion criteria (five studies); outcomes measured not in the inclusion criteria (four studies).