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What are filtered and unfiltered sources of evidence on the evidence pyramid?


Filtered and unfiltered resources differ in the level of evaluation applied to the works reviewed and the transparency of the authors regarding their evaluation methods, criteria for inclusion of articles, search strategies, and conflicts of interest of the review team members. The higher you go on the evidence pyramid, the higher the level of “filtering” and the greater one’s confidence can be in the strength and quality of the recommendations. Filtered evidence is the basis for both non-clinical policies, educational resources, and legislation and clinical practice guidelines, protocols, and care bundles.

**The level of “filtering” is not related to whether the document is published in a peer reviewed journal.

**Filtered sources are secondary sources.

**Filtered evidence synthesizes both qualitative and quantitative primary research articles.

**When you are asked in a research class to appraise research articles using some set of criteria, you are filtering the evidence!

Filtered resources are secondary sources. They include:

  • systematic reviews
  • critically appraised topics
  • clinical practice guidelines


Unfiltered resources are primary or original research studies. Unfiltered evidence sources are primary sources. Examples include:

  • randomized-controlled trails
  • cohort studies
  • case studies
  • descriptive and observational studies
  • qualitative and quantitative research articles

More information you can use is here:


Evidence Pyramid and Appraisal Tools

How do I find articles that are in specific levels of the evidence pyramid?

Reviews, Guidelines, Protocols, and Care Bundles, Oh My! 

How do I find qualitative or quantitative research articles?

What are primary or secondary sources?

What is peer review?



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