Registered Nurses' Association of Ontario

Levels of Evidence

Levels of evidence are assigned to study designs to rank how well particular designs are able to eliminate alternate explanations of the phenomena under study. The higher the level of evidence, the greater the likelihood that the relationships presented between the variables are true. Levels of evidence do not reflect the merit or quality of individual studies. Levels of Evidence


Evidence obtained from meta-analysis or systematic reviews of randomized controlled trials, and/or synthesis of multiple studies primarily of quantitative research.


Evidence obtained from at least one randomized controlled trial. IIa Evidence obtained from at least one well-designed controlled study without randomization.


Evidence obtained from at least one other type of well-designed quasi-experimental study, without randomization. III Synthesis of multiple studies primarily of qualitative research.


Evidence obtained from well-designed non-experimental observational studies, such as analytical studies or descriptive studies, and/or qualitative studies.


Evidence obtained from expert opinion or committee reports, and/or clinical experiences of respected authorities.

Adapted from the Scottish Intercollegiate Guidelines Network (2011) and Pati (2011).

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