Table 1.

Sackett's Initial Rules of Evidence, as Described by the Oxford Centre for Evidence-Based Medicine⁵⁴

Level of Evidence	Type of Study
I	(a) systematic review (homogeneous) of high quality RCTs
	(b) individual high-quality RCTs
	(c) all or none
II	(a) systematic review (homogeneous) of cohort studies
	(b) individual cohort study, including low-quality RCTs
	(c) “outcomes” research
III	(a) systematic review of case-control studies
	(b) individual case-control studies
IV	case series (no control group) and low-quality cohort and case-control studies.
V	expert opinion and single-subject design studies