Table 1.

Quality of evidence: number and level of evidence of peer-reviewed and published papers

Hierarchy of evidence
Level of evidence	Description	No. of studies
1 a	Systemic review (with homogeneity*) of randomized, controlled clinical trials (RCTs)	0
1 b	Individual randomized controlled clinical trial (RCT) (with narrow Confidence Interval‡)	0
1 c	All or none case series	0
2 a	Systemic review (with homogeneity*) of cohort studies	0
2 b	Individual cohort study (including low quality RCT; e.g., <80% follow-up)	0
2 c	“Outcomes” Research; Ecological studies	0
3 a	Systemic review (with homogeneity*) of case–control studies	0
3 b	Individual Case–control Study	23
4	Case-series (and poor quality cohort and case–control studies)	0
5	Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”	0
Others	Letters to editor, Abstract	0