Levels Of Evidence

Levels of Evidence - Relative to evidence-based practice, levels of evidence are an attempt to describe the strength of the results measured in a clinical trial or research study.  Note: levels of evidence do not supersede the critical evaluation of an experienced professional, and should be viewed as guidelines - a meta-analysis may be poorly constructed (Level IA), and a comparative study (Level III) may employ new technologies and a strong methodology. The Levels of Evidence used by the Brookbush Institute, as described by Shekelle et al.(1):
  • IA - Evidence from meta-analysis of randomized controlled trials
  • IB - Evidence from at least one randomized controlled trial
  • IIA - Evidence from at least one controlled study without randomization
  • IIB - Evidence from at least one other type of quasi-experimental study
  • III - Evidence from non-experimental descriptive studies, such as comparative studies, correlation studies, and case-control studies
  • IV - Evidence from expert committee reports or opinions or clinical experience of respected authorities, or both
  1. Shekelle, P. G., Woolf, S. H., Eccles, M., & Grimshaw, J. (1999). Developing clinical guidelines. Western Journal of Medicine, 170(6), 348.

Related Terms

  • Evidence-based practice
  • Meta-analysis
  • Randomized controlled trial
  • Quasi-experimental study
  • Descriptive study
  • Expert committee report
  • Clinical guidelines


  • level of evidence
  • evidence level
  • evidence levels