References	Risk of bias (RoB) domains a						Tier b
	Exposure KEY	Outcome KEY	Confounding KEY	Inappropriate selection	Attrition	Other sources of bias/statistics
Clarke et al. ( 2007 )	++	++	++	+	–	+	1
Morris et al. ( 2012)	+	++	++	++	NR	+	1
Doets et al. (2014)	+	+	++	+	NR	+	1

^a Expert judgement was translated into a rating scale for each question to be answered as follows: (++): definitely low RoB; (+): probably low RoB; (NR): not reported; (−): probably high RoB; (− −): definitively high RoB.

^b The individual rating for each question was combined by an algorithm and translated to an overall tier of reliability for each individual study (RoB tier 1: low RoB; RoB tier 2: moderate RoB; RoB tier 3: high RoB).