Table 3. Levels of evidence, adapted from OCEBM.

	Assessment during eligibility analysis	Increase during critical review	Decrease during critical review
Level 1	Systematic reviews of RCTs	Large or very large effect sizes.	Poor-quality, imprecise studies, lacking directionality, or with small effect sizes.
Level 2	RCTs or observational studies with dramatic effects
Level 3	Nonrandomized cohorts/follow-up studies
Level 4	Case series, case-control studies, controlled history studies
Level 5	Mechanism-based assessment

Abbreviations: OCEBM, Oxford Centre for Evidence-Based Medicine; RCT, Randomized clinical trial.