Registered Nurses' Association of Ontario

Levels of Evidence

Ia Evidence obtained from meta-analysis or systematic reviews of randomized controlled trials.
Ib Evidence obtained from at least one randomized controlled trial.
IIa Evidence obtained from at least one well-designed controlled study without randomization.
IIb Evidence obtained from at least one other type of well-designed quasi- experimental study, without randomization.
IV Evidence obtained from well-designed non-experimental descriptive studies, such as comparative  studies, correlation studies and case studies.
V Evidence obtained from expert committee reports or opinions and/or clinical experiences of respected authorities.


Adapted from “Annex B: Key to evidence statements and grades of recommendations,” by the Scottish Intercollegiate Guidelines Network (SIGN), 2012, in SIGN 50: A Guideline Developer’s Handbook. Available from

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