Level of evidence

Source of evidence

Level 1A

Evidence from systematic reviews and meta-analyzes of comparable clinical trials. Well-delineated randomized controlled trials with relevant clinical outcome.

Level 1B

Evidence based on randomized controlled trials with narrow confidence intervals.

Level 1C

Evidence of “all or nothing” results. Case-control study.

Level 2A

Evidence resulting from a homogeneous systematic review of cohort studies (with comparison groups and control variables).

Level 2B

Evidence based on cohort studies with poor quality of randomization, control or without long follow-up, cross-sectional cohort study.

Level 2C

Evidence of research results (observation of therapeutic results or clinical evolution).

Level 3A

Evidence resulting from homogeneous systematic review of case studies with control group.

Level 3B

Evidence based on case studies with control group.


Evidence from case and series reports without case-control definition.


Evidence from the opinion of authorities or experts. Review of non-systematic literature.