Table 1. Criteria for Each Level of Evidence.

I	Evidence obtained from meta‐analysis of multiple, well‐designed, controlled studies; randomized trials with low false‐positive and false‐negative errors (high power).
II	Evidence obtained from at least 1 well‐designed experimental study; randomized trials with high false‐positive and/or false‐negative errors (low power).
III	Evidence obtained from well‐designed, quasi‐experimental studies such as nonrandomized, controlled single‐group, pretest‐posttest comparison, cohort, time, or matched case‐control series.
IV	Evidence obtained from well‐designed, nonexperimental studies, such as comparative and correlational descriptive and case studies.
V	Evidence obtained from case reports and clinical examples.

Adapted from Somerfield MR, Padberg JR, Pfister DG, et al. ASCO clinical practice guidelines: process, progress, pitfalls, and prospects. Class Pap Curr Comments. 2000;4:881‐886.21