Hierarchies of evidence have been somewhat inflexibly used, and criticised, for some decades. The CEBM "levels of evidence" were first produced in 1998 for Evidence-Based On Call to make the process of finding appropriate evidence feasible and its results explicit. We have revised the "levels" in light of new concepts and data, and we would like to hear your feedback.
The "levels" are essentially a heuristic, or short-cut to finding the likely best evidence. While ideally we should look at "all the evidence" that might help to answer a question, doing so is often impractical or unfeasible. For example, suppose for a journal club next week you needed to look at the evidence for the use of warfarin in atrial fibrillation. A PubMed search of the words "atrial fibrillation AND warfarin" yields thousands of hits. As you will not have time to read them all, you will need some guidance as to which articles might be the most valid and useful for purpose. The table below shows a detailed breakdown, filtered by different MeSh terms for study types.
For example, if your main interest is the size of the benefit of warfarin and its common harms, it would be reasonable to focus on systematic reviews or trials. If no trials had been shown, you might look at the cohort studies, etc. Since there are several trials, it would be sensible to find out if a systematic review is being planned, and recommend or subsequently undertake one if there is not.
CEBM Levels of Evidence 2
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