AACN Levels of Evidence

Added to Collection

Level A — Meta-analysis of quantitative studies or metasynthesis of qualitative studies with results that consistently support a specific action, intervention, or treatment (including systematic review of randomized controlled trials).

Level B — Well-designed, controlled studies with results that consistently support a specific action, intervention, or treatment.

Level C — Qualitative studies, descriptive or correlational studies, integrative review, systematic review, or randomized controlled trials with inconsistent results.

Level D — Peer-reviewed professional and organizational standards with the support of clinical study recommendations.

Level E — Multiple case reports, theory-based evidence from expert opinions, or peer-reviewed professional organizational standards without clinical studies to support recommendations.

Level M — Manufacturer’s recommendations only.

(Excerpts from Peterson et al. Choosing the Best Evidence to Guide Clinical Practice: Application of AACN Levels of Evidence. Critical Care Nurse. 2014;34[2]:58-68.)

What is the purpose of levels of evidence (LOEs)?

“The amount and availability of research supporting evidence-based practice can be both useful and overwhelming for critical care clinicians. Therefore, clinicians must critically evaluate research before attempting to put the findings into practice. Evaluation of research generally occurs on 2 levels: rating or grading the evidence by using a formal level-of-evidence system and individually critiquing the quality of the study. Determining the level of evidence is a key component of appraising the evidence.1-3 Levels or hierarchies of evidence are used to evaluate and grade evidence. The purpose of determining the level of evidence and then critiquing the study is to ensure that the evidence is credible (eg, reliable and valid) and appropriate for inclusion into practice.3 Critique questions and checklists are available in most nursing research and evidence-based practice texts to use as a starting point in evaluation.”

How are LOEs determined?

“The most common method used to classify or determine the level of evidence is to rate the evidence according to the methodological rigor or design of the research study.3,4 The rigor of a study refers to the strict precision or exactness of the design. In general, findings from experimental research are considered stronger than findings from nonexperimental studies, and similar findings from more than 1 study are considered stronger than results of single studies. Systematic reviews of randomized controlled trials are considered the highest level of evidence, despite the inability to provide answers to all questions in clinical practice.”4,5

Who developed the AACN LOEs?

“As interest in promoting evidence-based practice has grown, many professional organizations have adopted criteria to evaluate evidence and develop evidence-based guidelines for their members.”1,5 Originally developed in 1995, AACN’s rating scale has been updated in 2008 and 2014 by the Evidence-Based Practice Resources Workgroup (EBPRWG). The 2011-2013 EBPRWG continued the tradition of previous workgroups to move research to the patient bedside.

What are the AACN LOEs and how are they used?

The AACN levels of evidence are structured in an alphabetical hierarchy in which the highest form of evidence is ranked as A and includes meta-analyses and meta-syntheses of the results of controlled trials. Evidence from controlled trials is rated B. Level C, the highest level for nonexperimental studies includes systematic reviews of qualitative, descriptive, or correlational studies. “Levels A, B, and C are all based on research (either experimental or nonexperimental designs) and are considered evidence. Levels D, E, and M are considered recommendations drawn from articles, theory, or manufacturers’ recommendations.”

“Clinicians must critically evaluate research before attempting to implement the findings into practice. The clinical relevance of any research must be evaluated as appropriate for inclusion into practice.”


  1. Polit DF, Beck CT. Resource Manual for Nursing Research: Generating and Assessing Evidence for Nursing Practice. 9th ed. Philadelphia, PA: Williams & Wilkins; 2012.
  2. Armola RR, Bourgault AM, Halm MA, et al; 2008-2009 Evidence-Based Practice Resource Work Group of the American Association of Critical-Care Nurses. Upgrading the American Association of Critical-Care Nurses’ evidence-leveling hierarchy. Am J Crit Care. 2009;18(5):405-409.
  3. Melnyk BM, Fineout-Overholt, E. Evidence-Based Practice in Nursing and Healthcare: A Guide to Best Practice. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2011.
  4. Gugiu PC, Gugiu MR. A critical appraisal of standard guidelines for grading levels of evidence. Eval Health Prof. 2010;33(3):233-255. doi:10.1177/0163278710373980.
  5. Evans D. Hierarchy of evidence: a framework for ranking evidence evaluating healthcare interventions. J Clin Nurs. 2003;12(1):77-84.