Table 1 Levels of Evidence
From: 2003 Canadian Asthma Consensus Guidelines Executive Summary
Level I | Evidence is based on randomized controlled trials (or meta-analysis of such trials) of adequate size to ensure a low risk of incorporating false-positive or false-negative results. |
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Level II | Evidence is based on randomized controlled trials that are too small to provide level I evidence. They may show either positive trends that are not statistically significant or no trends and are associated with a high risk of false-negative results. |
Level III | Evidence is based on nonrandomized controlled or cohort studies, case series, case-control studies, or cross-sectional studies. |
Level IV | Evidence is based on the opinion of respected authorities or expert committees as indicated in published consensus conference proceedings or guidelines. |
Level V | Evidence is based on the opinions of those who have written and reviewed the guidelines on the basis of experience, knowledge of the relevant literature, and discussion with peers. |