Table 1.

Categories of evidence levels

Category of evidence level	Description
Level I	Evidence from a systematic review or meta-analysis of randomized controlled trials (RCTs)
Level II	Evidence from 1 or more RCTs
Level III	Evidence from 1 or more non-randomized controlled studies
Level IVa	Evidence from 1 or more analytical epidemiology studies (cohort studies)
Level IVb	Evidence from 1 or more analytical epidemiology studies (case–control studies and/or cross-sectional studies)
Level V	Evidence from 1 or more descriptive studies (1 or more case reports and/or 1 or more case-series studies)
Level VI	Based on the opinion of experts (boards and/or individuals) without supportive patient data