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DNP Program: Evidence-Based Practice Resources: Evidence Levels/Study Designs

Levels of Evidence Explained

Video created by Humberto Reinoso
Video created by librarian at the University of Louisville Libraries

The creators of these videos produced them under a standard YouTube License. 

Levels of Evidence and Study Designs

Levels of quantitative research evidence are often illustrated by the evidence pyramid. There are many versions of the pyramid, but they all have filtered sources of information at the top. You may also sometimes see filtered sources referred to as synthesized sources because they synthesize single studies.  You should always try to find the highest level of evidence available for your clinical question. If you do not find the highest level, move down to the next level of the pyramid until you find relevant evidence.

This version of the Evidence Pyramid (developed at Tufts University) is based in part upon the Center for Evidence-Based Medicine (CEBM) Levels of Evidence and in part upon the Evidence Based Nursing Practice tutorial by Library Faculty at Scottsdale Community College. It gives greater detail on how the domain of the question being asked impacts article selection.

Evidence Pyramid” by Tufts University is reused (with no changes) under the BY-NC-SA license 4.0.

The description of the sources from which the pyramid was derived is quoted directly from Tufts University's "Selecting the Evidence" page.

6s Pyramid

The resources found in the 6S Pyramid contain evidence that will help you answer foreground questions (queries that bring together multiple concepts related to a specific clinical situation or research topic).

Six S Pyramid hierarchy of evidence; systems is the top, followed by summaries, synopses of syntheses, syntheses, synopses of single studies, with single studies at the bottom

[Adapted from DiCenso, Bayley and Haynes (2009). ACP Journal Club. Editorial: Accessing pre-appraised evidence: Fine-tuning the 5S model into a 6S model. Annals of Internal Medicine, 151(6):JC3-2, JC3-3.]

The 6S pyramid is arranged in a hierarchy, with the different levels outlined above.

Use the concepts identified in your PICO or PS Question (found in the Ask tab) to come up with approriate search terms, remembering:

  • A piece of evidence's ability to guide clinical action increases as you move up the pyramid.
  • Topics become more specific as you move down the pyramid.

This description of the 6s Model comes from McMaster University Health Sciences Library and is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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Evidence Sources by Category (based on the 6S Model or 6S Pyramid)

Systems, summaries, and synopses of summaries are relatively rare.  In most cases, they require subscription access, and are not available at Wright State.  Although syntheses (particularly systematic reviews) are being published much more frequently now and are easy to access through Wright State's subscriptions, they are not all of equal quality, so critical appraisal is necessary.


Integrating information from the lower levels of the hiearchy with individual patient records, systems represent the ideal source of evidence for clinical decision-making. 


Summaries are regularly updated clinical guidelines or textbooks that integrate evidence-based information about specific clinical problems.

Clinical Practice Guidelines

Evidence-Based Texts

Synopses of Syntheses

Synopses of syntheses, summarize the information found in systematic reviews.  By drawing conclusions from evidence at lower levels of the pyramid, these synopses often provide sufficient information to support clinical action.


Commonly referred to as a systematic review, a synthesis is a comprehensive summary of all the evidence surrounding a specific research question.   


Synopses of Single Studies

Synopses of single studies summarize evidence from high-quality studies.   Evidence-based abstract journals are the best place to find this type of information.

Single Studies

Article Databases


This description of the 6s Model categories with relevant links is adapted from McMaster University Health Sciences Library to address the availability of resources at Wright State. It is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Creative Commons License

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