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Critical Appraisal Resources for Evidence-Based Nursing Practice

Levels of Evidence

What are Levels of Evidence? 

  • In evidence-based practice, individual research studies are ranked and assigned levels of evidence based on their methodological rigor and strength
  • Study designs with the least potential risk for bias are at the top of the evidence hierarchy
  • Study designs with greater risk for bias are ranked lower
  • There are several variations of evidence hierarchy models. In most of these hierarchies, systematic reviews of randomized controlled trials are ranked highest, with expert opinion ranked lowest. 
  • Evidence levels are primarily an indicator of potential risk for bias being present in the study results, with the least risk for bias at the top and higher risk for bias at the bottom


Guyatt G. Rennie D. Meade M. Cook D. & American Medical Association. (2015). Users' guides to the medical literature: A manual for evidence-based clinical practice (3rd ed.). McGraw-Hill Education.

Melynk & Fineout-Overholt Levels of Evidence

In the field of nursing, the levels of evidence developed by Melnyk & Fineout-Overholt are the most universally utilized in evidence-based practice. 

Level 1: Systematic review & meta-analysis of randomized controlled trials; clinical guidelines based on systematic reviews or meta-analyses

Level 2: Randomized controlled trial

Level 3: Controlled trial (no randomization) [note: these include quasi-experimental studies]

Level 4: Case-control or cohort study

Level 5: Systematic review of descriptive & qualitative studies

Level 6: Single descriptive or qualitative study

Level 7: Expert opinion

Levels of Evidence - for Nursing Students at UToledo

Levels of evidence may vary slightly depending on which nursing research/EBP textbook you are using in your nursing program. 

Please consult your textbook or other professor-supplied resources for complete details on the levels of evidence that you are required to use. 

Copies of each nursing research/EBP text are available on Reserve from the Mulford Library. See below:  

Pre-Licensure BSN Program & RN-to-BSN Program

MSN, GEM, NP, and DNP Programs