TABLE 1.

Oxford University Center for Evidence-Based Medicine (OCEBM) evidence level and recommendation level standard.

Level of evidence	Definition
1a	Systematic review of randomized controlled trials (homogeneity)
1b	Individual randomized controlled trials (narrow confidence interval)
1c	When all patients died before the measure was introduced, but some patients now survive on it.
2a	Systematic review of cohort studies (homogeneity)
2b	Individual cohort studies (including low-quality randomized controlled trials; e g., follow-up rate < 80%)
2c	A study of the outcome; an ecological study
3a	Systematic review of case–control studies (homogeneity)
3b	Individual case-control study
4	Case series (and poor-quality cohort studies and case–control studies)
5	Lack of clear and strictly evaluated expert advice, or derived from physiology, laboratory research, or “first principles”
Level of recommended	Definition
A	Evidence of consistent level 1
B	Consistent level 2 or 3 evidence, or extrapolation based on level 1 evidence. (“extrapolation” means that data are applied to situations with potentially clinically important differences rather than the original research)
C	Level 4 evidence, or extrapolation based on level 2 or 3 evidence
D	Level 5 evidence, either inconsistent or inadequate research (any level)

OCEBM, Oxford University Center for Evidence-Based Medicine.