"Levels of Evidence" tables have been developed which outline and grade the best evidence. However, the review question will determine the choice of study design.
Secondary sources provide analysis, synthesis, interpretation and evaluation of primary works. Secondary sources are not evidence, but rather provide a commentary on and discussion of evidence. e.g. systematic review
Primary sources contain the original data and analysis from research studies. No outside evaluation or interpretation is provided. An example of a primary literature source is a peer-reviewed research article. Other primary sources include preprints, theses, reports and conference proceedings.
Levels of evidence for primary sources fall into the following broad categories of study designs (listed from highest to lowest):
Based on information from Centre for Reviews and Dissemination. (2009). Systematic reviews: CRD's guidance for undertaking reviews in health care. Retrieved from http://www.york.ac.uk/inst/crd/index_guidance.htm
"Levels of Evidence" are often represented in as a pyramid, with the highest level of evidence at the top:
The following definitions are adapted from the Glossary in "Systematic reviews: CRD's Guidance for Undertaking Reviews in Health Care", Centre for Reviews and Dissemination, University of York:
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