Table 2

Results of the assessment of risk of bias observed in the included studies (see also table 1) 26

	Major domains of bias					Minor domains of bias
	Exposure definition	Exposure assessment	Reliability of exposure assessment	Analysis /research specific bias	Confounding	Attrition	Blinding of assessors	Selective reporting	Funding	Conflict of interest	nrLR major	nrLR minor	HR/LR
Andersen et al1	LR	LR	UR	LR	LR	LR	HR	LR	LR	UR	4	3	LR
Harkness et al22	LR	LR	UR	LR	LR	HR	HR	LR	LR	UR	4	2	LR
Hoozemans et al23	LR	LR	LR	LR	LR	HR	LR	LR	UR	UR	5	2	LR
Hughes et al31	HR	HR	HR	HR	HR	HR	HR	LR	UR	UR	0	1	HR
Seidler et al11 33 34	HR	HR	UR	LR	LR	HR	HR	LR	UR	LR	2	2	HR
Smedley et al9	LR	HR	UR	LR	UR	HR	HR	LR	LR	UR	2	2	HR
Van der Beek et al32	HR	HR	HR	HR	HR	HR	HR	LR	UR	UR	0	1	HR

HR, high risk of bias; HR/LR, final score of study for high risk (HR) or low risk (LR) of bias;
LR, low risk of bias; nrLR major, number of domains with LR in the major 5 domains; nrLR minor, number of domains with LR in the minor 5 domains; UR, unclear risk.