Levels of Research and Quality of Evidence

OT Dude Academy’s Free NBCOT® Exam Prep Course Research Methods and Psychometrics Levels of Research and Quality of Evidence

These are often depicted as pyramids and are hierarchical.

Quality of Evidence1 

(highest to lowest)

  1. Systematic reviews
  2. Randomized controlled trials (RCTs)
  3. Cohort studies
  4. Case-control studies
  5. Case series, Case reports
  6. Editorials, Expert opinion

Another version of the pyramid includes CAPs/CATs:2 

  1. Systematic reviews – authors have systematically searched, appraised, and summarized the literature for a topic
  2. Critically-appraised topics (CATs) – authors critically appraised the topics to evaluate and synthesize numerous research studies
  3. Critically-appraised individual articles – authors critically appraised individual articles to evaluate and synthesize individual research studies.
  4. Randomized controlled trials (RCTs) – include a random group of participants in an experimental group and control group and have multiple variables followed-up on.
  5. Cohort studies – identifies two groups of participants (cohort) and follows them in the study
  6. Case-controlled studies/Case series, Case report – involves identifying participants who have outcomes of interest (cases) and control participants without the same outcome to see if they had the exposure of interest.
  7. Expert opinion, Background information – handbooks, encyclopedias, textbooks

Levels of Evidence3 

(High to low)

  • Level I
    • Level IA – Systematic review of RCTs
    • Level IB – Individual RCTs
  • Level II
    • Level IIA – Systematic reviews of cohort studies
    • Level IIB – Individual cohort studies
    • Level IIC – Outcome studies, Ecological studies
  • Level III
    • Level IIIA – Systematic review of case-control studies
    • Level IIIB – Individual case-control studies
  • Level IV – Case series
  • Level V – Expert opinions
  • Foundational Evidence – animal research, in vitro research, ideas, speculation
  1. Daly J, Willis K, Small R, Green J, Welch N, Kealy M, et al. A hierarchy of evidence for assessing qualitative health research. Journal of Clinical Epidemiology. 2007;60(1):43-9. 10.1016/j.jclinepi.2006.03.014.[↩]
  2. Greenhalgh, Trisha. How to Read a Paper: the Basics of Evidence Based Medicine. London: BMJ, 2000.
    Glover, Jan; Izzo, David; Odato, Karen & Lei Wang. EBM Pyramid. Dartmouth University/Yale University. 2006.[↩]
  3. González-García, Raúl. (2019). Scientific evidence in surgery for the treatment of temporomandibular joint internal derangement. Stomatological Disease and Science. 2019. 10.20517/2573-0002.2018.26.[↩]
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