Level of Evidence	Criteria for analysis and inclusion
I	Meta-analyses 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-control
II	Cohort studies or randomized case-control 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 with reasonable probability of causation
IV	Non-analytic studies as case series or individual cases