Table 1.

Classification of scientific evidence in systematic literature reviews according to the evidence levels recommended by the Oxford Center for Evidence-Based Medicine.

Level of evidence	Grading criteria
1a	Systematic reviews of RCTs including meta-analysis.
1b	Individual RCT with narrow confidence interval.
1c	Case of series “all or nothing.”
2a	Systematic review of cohort studies.
2b	Individual cohort study and low quality RCT.
2c	Outcome research study.
3a	Systematic review of case control studies.
3b	Individual case control study.
4	Case series, poor quality cohort, and case control studies.
5	Expert's opinion.

Adapted from levels of evidence of the Oxford Center for Evidence-Based Medicine [62].