References	Risk of bias domainsa								Tierb
	Randomisation	Allocation concealment	Blinding	Attrition	Exposure	Outcome	Reporting	Other threats to internal validity
Miller et al. (1988)	+	NR	−	+	++	+	+	+	1
He et al. (2015)	++	++	+	++	++	−	++	+	2

^aExpert 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.

^bThe 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).