Table 1.

Level and Grades for Evidence-based Recommendations

Level	Definition	Grade	Definition
1	SR (with homogeneity) of RCTs	A	Consistent level 1 studies
1b	Individual RCT (with narrow “confidence interval”)
1c	All or nonea
2	SR (with homogeneity) of cohort studies	B	Consistent level 2 or3w
2b	Individual cohort study (including low-quality RCT; eg, <80% follow-up)
2c	“Outcomes” research; ecological studies
3	SR (with homogeneity) of case-control studies
3b	Individual case-control study
4	Case series (and poor quality cohort and case-control studies)	C	Level 4 studies or extrapolations from level 2 or 3 studies
5	Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”	D	level 5 evidence or troublingly inconsistent or inconclusive studies of any level

^aMet when all patients died before the treatment became available, but some now survive on it; or when some patients died before the treatment became available, but none now die on it.

RCT = randomized controlled trials; SR = systematic review.

Adapted from the Oxford Centre for Evidence-Based Medicine: Levels of Evidence.³⁷