Table 2.

Aggregate grades of evidence by question type

Grade	OCEBM Level	Treatment	Harm
A	1	Systematic review of randomized trials	Systematic review of randomized trials, nested case-control studies, or observational studies with dramatic effect
B	2	Randomized trials or observational studies with dramatic effects or highly consistent evidence	Randomized trials or observational studies with dramatic effects or highly consistent evidence
C	3–4	Nonrandomized or historically controlled studies, including case-control and observational studies	Nonrandomized controlled cohort or follow-up study (post-marketing surveillance) with sufficient numbers to rule out a common harm, case-series, case-control, or historically controlled studies
D	5	Case reports, mechanism-based reasoning, or reasoning from first principles
X	NA	Exceptional situations where validating studies cannot be performed and there is a clear preponderance of benefit over harm

OCEBM, Oxford Centre for Evidence-Based Medicine; NA, not applicable.