Levels of Evidence | Type of Evidence |
---|---|
Ia | Systematic review (with homogeneity)b of level-1 studiesc |
Ib | Level-1 studiesc |
II | Level-2 studiesd |
Systematic reviews of level-2 studies | |
III | Level-3 studiese |
Systematic reviews of level-3 studies | |
IV | Consensus, expert committee reports or opinions and/or clinical experience without explicit critical appraisal; or based on physiology, bench research, or “first principles” |
↵a Adapted from The Oxford Centre for Evidence-Based Medicine Levels of Evidence (2001) and the Centre for Reviews and Dissemination Report Number 4 (2001). Copyright National Institute for Health and Care Excellence February 2004.
↵b Homogeneity means there are no or minor variations in the directions and degrees of results between individual studies that are included in the systematic review.
↵c Level-1 studies are studies:
• that use a blind comparison of the test with a validated reference standard.
• in a sample of patients that reflects the population to whom the test would apply.
↵d Level-2 studies are studies that have only 1 of the following:
• narrow population (the sample does not reflect the population to whom the test would apply).
• use a poor reference standard (defined as that where the “test” is included in the “reference,” or where the “testing” affects the “reference”).
• the comparison between the test and reference standard is not blind.
• case-control studies.
↵e Level-3 studies are studies that have at least 2 or 3 of the features listed above.