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Nursing 472: Levels of Evidence

Hierarchy of Evidence

You will see many different versions of the evidence hierarchy pyramid.  However, they all have commonalities:

Meta-Analysis / Meta-Synthesis / Systematic Review: These are sources of high level evidence as they are analyses of existing research. They are generally at the top of the evidence pyramid. Be sure to look at inclusion/exclusion criteria and forest plots to appraise the quality of the source.

Clinical Practice Guideline (CPG): CPGs are also high level evidence. They are put in place by those who have analyzed existing research on a topic in order to develop the guideline.

- Randomized Controlled Trial (RCT): RCTs are also high quality evidence. RCTs are often ranked right below meta analyses, meta syntheses, systematic reviews, and CPGs. Randomization increases the quality of the source and reduces bias.

- Experimental Study: Experimental studies rank below RCTs, but are still good sources of evidence. In order to have an experiment you must have control, manipulation, and randomization.

- Non-experimental/Descriptive/Correlational: These rank below experimental studies, but above qualitative studies.

- Qualitative Study: Qualitative studies often have fewer participants and are often focused on the experience of something.  Although qualitative studies may not be at the top of the pyramid, they are very important to some topics. For instance, a qualitative study would be very helpful for topics such as the experience of having a premature birth or the experience of going through cancer treatments.

- Expert Opinion: Expert opinions are lower level evidence.

Primary Research

Primary research articles are written by those who did the experiment.   Sometimes other publications are written about primary research, but an article is only primary research if it is the actual publication written by the people who did the experiment.

Things to look for:

  • Article describes a study/intervention (ex: case study, cohort study, randomized controlled trial, etc.)
  • Article authors conducted the study and write about it
  • Can be quantitative (looking at numbers) or qualitative (looking at themes)


Not a primary research article if:

  • Article talks about study, but article author did not conduct study (ex: news report about study findings)
  • Authors don't conduct an experiment (ex: editorials, opinions)
  • Authors collect and describe other authors' work (ex: review article, systematic review, meta-analysis, meta-synthesis, clinical practice guideline)

Tutorial on the difference between Primary and Secondary Research articles.

Example studies

Health Sciences Librarian

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Juliet Kerico Gray -- Health Sciences
Lovejoy Library, Office 1054


Guide developed with assistance from S. Pruitt, student, SIUE School of Nursing, and 2014 URCA program assistant.