Registered Nurses' Association of Ontario

Interpretation of Evidence

Levels of evidence are assigned to study designs to rank how well each study design is able to eliminate alternate explanations of the phenomena under study. The higher the level of evidence, the more likely it is that there were fewer potential sources of bias influencing the research findings. However, levels of evidence do not reflect the quality of individual studies or reviews.

In some cases, recommendations in this BPG are assigned more than one level of evidence. This reflects the varied study designs that support the recommendation. For transparency, the level of evidence for each component of the recommendation statement is identified.

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

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.

III: Synthesis of multiple studies primarily of qualitative research.

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

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


Quality of Evidence

In addition to the levels of evidence, the quality of each of the reviews cited in the discussion of evidence was appraised and categorized as strong, moderate, or low based on the AMSTAR instrument for reviews. The quality rating is calculated by converting the score on the AMSTAR tool into a percentage. When other guidelines informed the recommendation and discussion of evidence, the AGREE II instrument was used to determine the quality rating. The tables below highlight the quality scores required to achieve a strong, moderate, or low quality rating.


Quality Rating for Reviews Using the AMSTAR Tool*

Quality Score on the AMSTAR

Overall Quality Rating

Greater than, or equal to, a converted score of 82.4%


A converted score of 62.5 – 82.4%


Less than, or equal to, a converted score of 62.4%



* It is important to recognize that the overall quality of the systematic review may be low, but that the primary studies included in the review may have a higher quality rating if appraised individually.

Quality Rating for Guidelines Using the AGREE II Tool

Quality Score on the AGREE II

Overall Quality Rating

A score of 6 or 7 on the overall guideline quality


A score of 5 on the overall guideline quality


A score of less than 4 on the overall guideline quality

(Not used to support recommendations)


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