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Levels of Evidence
- The definition of Evidence according to The Users’
Guide*: “Any empirical observation about apparent relation between
events constitutes potential evidence.”
- Included in this definition are unsystematic observations
of an individual clinician.
- Some unsystematic observations from experienced clinicians
can provide profound insight and can be a starting point for more
rigorous examination. Unsystematic observations are also limited in
many ways. For example, observational studies tend to overestimate
treatment effects.
- Physiologic experiments or extrapolations are another
source of evidence, which usually lead to sound clinical outcomes
but are occasionally disastrously wrong. Therefore, EBM suggests a
hierarchy of Evidence, from the least biased (highest) to the most
biased (lowest).
- The EBM process ensures a physician is using the highest
(best) level of evidence available to make clinical decisions. Certainly
the sum total of all medical knowledge is far from complete, and often
the only thing available to a physician in a given clinical area is
his or her own experience and judgment. However, substituting anecdotal
experience in areas where there is good, scientific evidence is not
practicing at the state of the art.
- *The Users’ Guides to the Medical Literature:
Essentials of Evidence-Based Clinical Practice. Edited by Gordon
Guyatt and Drummond Rennie, AMA Press. 2002.
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