The "evidence pyramid" is often used to illustrate the levels of evidence in the literature. When beginning your search for evidence, begin at the highest possible tier.
Filtered information is "pre-appraised." This means that the content has been filtered to include studies and reviews that are of higher quality. Keep in mind that the amount of available literature and the number of problems covered gets smaller as you move up the pyramid.
Unfiltered information represents the original studies. These tiers may not contain studies of high quality and strong evidence, but they cover a much broader range of clinical problems and are much more available.
If you don't know where to begin, consider using a search engine that simultaneously searches multiple websites to find the evidence.
EBM Pyramid and EBM Page Generator, ©2006 Trustees of Dartmouth College and Yale University.All rights reserved. Produced by Jan Glover, David Izzo, Karen Odato and Lei Wang.
These search engines simultaneously search multiple websites to find the evidence to help you answer your clinical question.
Authors of a systematic review ask a specific clinical question, perform a comprehensive literature search, eliminate the poorly done studies and attempt to make practice recommendations based on the well-done studies. A meta-analysis is a systematic review that combines all the results of all the studies into a single statistical analysis of results.
Authors of critically-appraised topics evaluate and synthesize multiple research studies.
Authors of critically-appraised individual articles evaluate and synopsize individual research studies.
Evidence is not always available via filtered resources. Searching the primary literature may be required. It is possible to use specific search strategies in PubMed (Medline) and other databases to achieve the highest possible level of evidence. Otherwise, consider filtering the search to specific types of studies. See the Glossary of Studies box for suggestions.
Note: Evidence in these resources may vary from expert opinion to high levels of evidence.
Definitions of selected types of studies are given below. For additional definitions, consult the references listed below.
A detailed report of the diagnosis, treatment, and follow-up of an individual patient. Case reports also contain some demographic information about the patient (for example, age, gender, ethnic origin).
A group or series of case reports involving patients who were given similar treatment. Reports of case series usually contain detailed information about the individual patients. This includes demographic information (for example, age, gender, ethnic origin) and information on diagnosis, treatment, response to treatment, and follow-up after treatment.
A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls). Researchers study the medical and lifestyle histories of the people in each group to learn what factors may be associated with the disease or condition. For example, one group may have been exposed to a particular substance that the other was not. Also called retrospective study.
A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition.
Randomized Controlled Trial
A controlled clinical trial that randomly (by chance) assigns participants to two or more groups. There are various methods to randomize study participants to their groups.
Agency for Healthcare Research and Quality. Glossary of terms. Retrieved from: http://www.effectivehealthcare.ahrq.gov/index.cfm/glossary-of-terms/
Note: Glossary of terms is no longer available.
National Cancer Institute. Dictionary of cancer terms. Retrieved from: http://www.cancer.gov/dictionary