Table 2.

Classification of levels of evidence

LEVEL	TYPES OF EVIDENCE
I	High-quality RCT with statistically significant difference or no statistically significant difference but narrow CIs
	Systematic review of level I RCTs (and study results were homogenous)
II	Lower-quality RCT (eg, < 80% follow-up, no blinding, or improper randomization)
	Prospective comparative study
	Individual cohort study
	Systematic review of level II studies or level I studies with inconsistent results
III	Case-control study
	Retrospective comparative study
	Systematic review of level III studies
IV	Case series
V	Expert opinion

CI—confidence interval, RCT—randomized controlled trial.

Adapted from the Oxford University Centre for Evidence-Based Medicine.⁷