Levels of Evidence
Various groups have established levels or hierarchies of evidence, usually based upon scientific merit in an empirical model. These hierarchies assist the clinician to rate a body of knowledge by evaluating the strength of types of evidence under review (Schmidt & Brown, 2009).
- After the sources of evidence (clinician experience, patient preferences, and scientific findings) have been found, each source must be evaluated for its strength.
- Strength is evaluated by asking: "how good is the evidence (meaning how valid and relevant) for this particular treatment-related clinical situation?"
- To answer the "how good" question, researchers have created typologies of hierarchical evidence strength.
Evidence can include a variety of research and non-research sources, such as integrative reviews, research studies, quality-improvement data, and case studies. These sources can be further categorized into levels. One example of an evidence categorization model was developed by Melnyk and Fineout-Overholt (2005). It includes seven levels and rates them from strongest to weakest. The stronger the evidence, the higher the likelihood that it is valid and relevant for a particular clinical situation. Therefore, in clinical practice, stronger types of evidence would be the first choice and carry a higher chance of showing efficacy for a particular problem.
Melnyk's Hierarchy of Evidence
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Level
|
Description
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Strength
|
|
I
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Evidence from a systematic review or meta-analysis of all relevant randomized controlled trials (RCTs), or evidence-based clinical practice guidelines based on systematic reviews of RCTs
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Strongest |
|
II
|
Evidence from at least one well-designed RCT
|
|
|
III
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Evidence from well-designed controlled trials without randomization
|
|
|
IV
|
Evidence from well-designed case-control and cohort studies
|
|
|
V
|
Evidence from systematic reviews of descriptive and qualitative studies
|
|
|
VI
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Evidence from a single descriptive or qualitative study
|
|
|
VII
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Evidence from the opinion of authorities and/or reports of expert committees
|
Weakest
|
Note. Based on information from Melnyk & Fineout-Overholt, 2005.
Additional evidence hierarchies and recommendations include:
Continue to Step 3.3 - Credibility of Websites