Table 2.

Classification of levels of evidence

Levels of evidence	Criteria for analysis and inclusion
I	Meta-analysis and systematic reviews of randomized, controlled trials (RCTs) of high quality, or RCTs with minimum or low risk of bias. Systematic reviews of high quality relative to cohort studies or case–controls.
II	Cohort studies or randomized case–controls of high quality with minimal risk of confounding or bias and with high or discrete probability of causation.
III	Case–control studies and retrospective comparison of well-conducted studies with reasonable probability of causation.
IV	Non-analytic studies as case series or individual cases.