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.
4	Evidence from case and series reports without case-control definition.
5	Evidence from the opinion of authorities or experts. Review of non-systematic literature.