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PHM101 Research Skills Guide: Evidence-Based Information

Levels of Evidence - Secondary and Primary

There are two broad types of evidence: secondary and primary.

We list secondary first because in Evidence-Based Practice it is the higher level of evidence and will probably be what you seek first in answering a clinical or research question.

Secondary evidence (filtered, pre-appraised)

This is evidence assimilated, or put together, from a number of quality primary studies on a topic. It includes systematic reviews, meta-analyses, and evidence summaries.

This is sometimes referred to as filtered or pre-appraised evidence.

You can find it in specialised EBP sources such as The Cochrane Library (notably in the Cochrane Database of Systematic Reviews) and in health and medical databases such MEDLINE, PubMed and CINAHL Plus with Full Text. See Where to Search for Evidence, for details of where to search for secondary, filtered, pre-appraised evidence.


Primary evidence (unfiltered)

This body of evidence consists of original individual studies such as controlled trials, cohort studies, and case studies.

This is sometimes referred to as unfiltered evidence.

You can find primary studies in specialised EBP resources such as The Cochrane Library, notably in the Central Register of Controlled Trials, and in health and medical databases. See the next page for details of where to search for primary unfiltered evidence.

An Introduction to the Cochrane Library

For more information please see: How to use the Cochrane Library

Further Reading

For more information on evidence-based practice and details of where to search please see our Evidence-Based Practice Guide

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