Table 1.

Risk of bias

Study	Selection bias	Allocation concealment bias	Performance bias	Detection bias	Attrition bias	Selective reporting	Other bias	Overall risk
McCowan et al.28	No	No	No	Unclear: blinding of outcome assessors not detailed	Yes: attrition rate variation was not fully explained. No intention-to-treat analysis	Unclear: no protocol	No	C-High
Eccles et al.29	No	No	No	No	No	No	No	A-Low
Zeiger et al.30	No	No	No	No	No	No	No	A-Low
Tamblyn et al.31	No	No	No	No	No	No	No	A-Low