Not all evidence is equal. Presented below are three pyramids that show the different levels of evidence sources and explanations of each level.Of course, the best are at the top and as the pyramids indicate, you have much fewer resources at the top than at the bottom of the pyramids.
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Hierarchy of Evidence
Not all evidence is judged to be of equal value; that is, there are hierarchies of research design that are evaluated to have different strengths or different levels of value in the decision making process. See the two charts below -- one graphically represented, one textually to help understand the concepts important to critical appraisal, assessment, and evaluation of research.
The Evidence Pyramid
[Source: SUNY Downstate Medical Center. Medical Research Library of Brooklyn. Evidence Based Medicine Course. A Guide to Research Methods: The Evidence Pyramid.]
Levels of Evidence
| Category I | Evidence from at least one properly randomized controlled trial. |
| Category II-1 | Evidence from well-designed controlled trials without randomization. |
| Category II-2 | Evidence from well-designed cohort or case-control analytic studies preferably from more than one center or research group. |
| Category II-3 | Evidence from multiple times series with or without intervention or dramatic results in uncontrolled experiments such as the results of the introduction of penicillin treatment in the 1940s. |
| Category III | Opinionsof respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees. |
[Source: Harris, R.P. et al. [2001]. Current methods of the U.S. Preventive Services Task Force: a review of the process. American Journal of Preventive Medicine. April 20 [3 Supplement]: 21-35.]
Another way to look at information evidence:
The EVIDENCE PYRAMID is often used to illustrate the development of evidence. At the base of the pyramid is animal research and laboratory studies - this is where ideas are first developed. As you progress up the pyramid the amount of information available decreases in volume, but increases in relevance to the clinical setting.
| Meta Analysis: | systematic review that uses quantitative methods to synthesize and summarize the results. |
| Systematic Review: | summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies and that uses appropriate statistical techniques to combine these valid studies. |
| Randomized Controlled Trial: | participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest. |
| Cohort Study: | involves identification of two groups [cohorts] of patients, one which received the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest. |
| Case Control Study: | study which involves identifying patients who have the outcome of interest [cases] and patients without the same outcome [controls], and looking back to see if they had the exposure of interest. |
| Case Series: | report on a series of patients with an outcome of interest. No control group is involved. |
[Definitions from CEBM]