Table 2.

Criteria for quality of the included studies and best-evidence synthesis

Item	Level	Criteria for inclusion
Level of studies	High-quality studies	Multivariate analysis performed and had a quality score ≥8
	Moderate-quality studies	Multivariate analysis performed but had a quality score <8 or
		No multivariate analysis performed and had a quality score ≥5
	Low-quality studies	No multivariate analysis performed and had a quality score <5
Level of evidence	Strong evidence	Minimum of three high-quality studies with generally consistent findings
	Moderate evidence	Minimum of two moderate-quality studies with generally consistent findings
	Limited evidence	Minimum of one low-quality study with generally consistent findings
	Conflicting evidence	Converse findings in >50 % of the studies
	No evidence	No study could report a positive correlation between certain factors and outcomes