Table 1.

Level of evidence

Level	Type of evidence
IA	Systematic review (with homogeneity) of RCTs
IB	Individual RCT (with narrow CI)
IC	All or none RCT
IIA	Systematic review (with homogeneity) of cohort studies
IIB	Individual cohort study (including low quality RCT, e.g., < 80% of follow-up)
IIC	“Outcomes” research; ecological studies
IIIA	Systematic review (with homogeneity) of case–control studies
IIIB	Individual case–control study
IV	Case series (poor quality cohort and case–control study)
V	Expert opinion without explicit critical appraisal or based on physiological bench research or “first principles”

RCT randomized controlled trial, CI confidence interval