Table 2.

Risk of Bias of Included Studies.

Authors	Selection	Comparability	Outcome	Overall quality
Airoldi et al 13	**	*	*	**/*/*
Borggreven et al 14	***	*	**	***/*/**
Bozec et al 15	****	*	**	****/*/**
Bozec et al 16	****		**	****/ /**
Bozec et al 17	**	*	**	**/*/**
Bozec et al 18	**		**	**/ /**
Chang et al 19	***		**	***/ /**
Dimovska et al 20	**	*	**	**/*/**
Elfring et al 21	***	*	*	***/*/*
Hartl et al 22	**		**	**/ /**
Jimenez et al 23	****	*	**	****/*/**
Klug et al 24	***	*	*	***/*/*
Lahtinen et al 25	****	*	**	****/*/**
Markkanen-Leppänen et al 26	**		*	**/ /*
Momeni et al 27	**		**	**/ /**
Oskam et al 28	***	*	*	***/*/*
Pierre et al 29	***	*	**	***/*/**
Rhemrev et al 30	****	*	**	****/*/**
Segna et al 31	**	*	*	**/*/*
Smith et al 32	***	*	*	***/*/*
Tamer et al 33	***	*	**	***/*/**
Yang et al 34	**		**	**/ /**

^*Each asterix represents the number of points accorded in each category. A maximum of 4 points could be given in the Selection category, a maximum of 2 points could be given in the Comparability category and a maximum of 3 points could be given in the Outcome category. Overall quality represents the number of points given in each category : Selection / Comparability / Outcome.