Table 2.

Risk of bias assessment of the included studies.

Guideline	Checklist	Aragón et al., 2018	Holmer et al., 2018	Cestari et al., 2016	Noble et al., 2014	Martande et al., 2014	Sheu and Lin, 2013	Stein et al., 2012	Müller et al., 2011	Syrjälä et al., 2012	Einarsdóttir et al., 2009	Hanaoka and Kashihara, 2009	Kamer et al., 2009
Study design appropriate to objectives?	Objective common design
	Prevalence cross-sectional
	Prognosis cohort
	Treatment controlled trial
	Cause cohort, case–control, cross-sectional	0	0	0	0	0	0	0	0	0	0	0	0
Study sample representative?	Source of sample	0	0	0	0	0	0	0	0	0	+	0	0
	Sampling method	+	0	+	+	+	+	0	0	0	+	+	+
	Sample size	0	0	+	0	0	0	0	0	0	0	0	++
	Entry criteria/exclusion	0	0	0	0	0	0	0	0	0	0	0	0
	Non-respondents	0	0	0	0	0	0	0	0	0	0	0	0
Control group acceptable?	Definition of controls	0	0	0	0	0	0	0	0	0	0	0	0
	Source of controls	0	0	0	0	0	0	0	0	0	0	0	0
	Matching/ randomization	0	0	0	0	0	0	0	0	0	+	0	0
	Comparable characteristics	0	0	0	0	0	0	0	0	0	0	0	0
Quality of measurements and outcomes?	Validity	0	0	0	+	0	0	+	0	0	+	0	+
	Reproducibility	0	0	0	0	0	0	0	0	0	+	0	+
	Blindness	++	++	++	++	0	++	++	++	++	++	++	++
	Quality control	+	+	0	+	0	0	+	0	0	+	0	+
Completeness	Compliance	0	0	0	0	0	0	0	0	0	0	0	0
	Drop outs	0	0	0	0	0	0	0	0	0	0	0	0
	Deaths	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA
	Missing data	0	0	0	0	0	0	0	0	0	0	0	0
Distorting influences?	Extraneous treatments	0	0	0	0	0	0	0	0	0	0	0	0
	Contamination	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA
	Changes over time	0	0	0	0	0	0	0	0	0	0	0	0
	Confounding factors	0	+	0	+	+	0	+	+	+	+	+	+
	Distortion reduced by analysis	0	0	+	0	0	0	0	+	0	+	0	0
Summary questions	Bias: Are the results erroneously biased in a certain direction?	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO
	Confounding: Are there any serious confusing or other distorting influences?	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO
	Chance: Is it likely that the results occurred by chance?	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO	NO

0, no problem; +, minor issues; ++, major problems; NA, not applicable.