Table 2.

Quality assessment of methods and risk of bias for selected studies.

Guideline	Checklist	Sakki et al., 1995	Merchant et al., 2003	Al-Zahrani et al., 2005b	Al-Zahrani et al., 2005a	Sanders et al., 2009	Bawadi et al., 2011	Samnieng et al., 2013
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
Study sample representative?	Source of sample	0	++	0	0	0	0	0
	Sampling method	++	++	++	+	0	0	0
	Sample size	+	+	+	0	0	+	0
	Entry criteria/exclusion	0	0	0	0	0	0	0
	Non-respondents	0	0	0	0	0	0	0
Control group acceptable?	Definition of controls	0	0	0	0	0	0	0
	Source of controls	0	0	0	0	0	0	0
	Matching/randomization	0	+	+	0	+	0	0
	Comparable characteristics	0	0	0	0	0	0	0
Quality of measurements and outcomes?	Validity	0	0	0	0	0	0	0
	Reproducibility	0	0	0	0	0	0	0
	Blindness	0	0	0	0	0	0	0
	Quality control	+	+	0	0	0	0	0
Completeness	Compliance	0	0	0	0	0	0	0
	Drop outs	0	0	0	0	0	0	0
	Deaths	0	0	0	0	0	0	0
	Missing data	0	0	0	0	0	0	0
Distorting influences?	Extraneous treatments	0	0	0	0	0	0	0
	Contamination	0	0	0	0	0	0	0
	Changes over time	0	0	0	0	0	0	0
	Confounding factors	0	0	0	0	0	0	0
	Distortion reduced by analysis	0	0	0	0	0	0	0
Summary questions	Bias:
	Are the results erroneously biased in certain direction?	NO	NO	NO	NO	NO	NO	NO
	Confounding:
	Are there any serious confusing or other distorting influences?	NO	NO	NO	NO	NO	NO	NO
	Chance:
	Is it likely that the results occurred by chance?	NO	NO	NO	NO	NO	NO	NO