Levels of evidence

Level Type of evidence (based on AHCPR 1992)


  • Ia Evidence obtained from meta-analysis of randomised controlled trials
  • Ib Evidence obtained from at least one randomised controlled trial


  • IIa Evidence obtained from at least one well designed controlled study without randomisation
  • IIb Evidence obtained from at least one other type of well designed quasiexperimental study


  • III Evidence obtained from well designed non-experimental descriptive studies, such as comparative studies, correlation studies, and case-control studies


  • IV Evidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities


Grading of recommendations

Grade Recommendation (based on AHCPR 1994)

Grade A

  • A (evidence levels Ia, Ib) Requires at least one randomised, controlled trial as part of the body of literature of overall good quality and consistency addressing the specific recommendation.

Grade B

  • B (evidence levels IIa,IIb, III) Requires availability of well conducted clinical studies but no randomised clinical trials on the topic of recommendation

Grade C

  • C (evidence level IV) Requires evidence from expert committee reports or opinions and/or clinical experience of respectedauthorities. Indicates absence of directly applicable studies of good quality


Halperin JL, Levine GN, Al-Khatib SM, Birtcher KK, Bozkurt B, Brindis RG,Cigarroa JE, Curtis LH, Fleisher LA, Gentile F, Gidding S, Hlatky MA, Ikonomidis J, Joglar J, Pressler SJ, Wijeysundera DN. Further Evolution of the ACC/AHA Clinical Practice Guideline Recommendation Classification System: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines. J Am Coll Cardiol. 2016 Apr 5;67(13):1572-1574. doi: 10.1016/j.jacc.2015.09.001. Epub 2015 Sep 24. PubMed PMID: 26409257.
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