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Nursing - Systematic Reviews: Levels of Evidence

Nursing: systematic reviews

"How would I use the 6S Model while taking care of a patient?"

Polit–Beck Evidence Hierarchy/Levels of Evidence Scale for Therapy Questions

VIIILevel VIII: Nonresearch source (e.g., internal evidence,Level VII: Qualitative study/descriptive studyLevel VI: Systematic review/metasynthesis of qualitative studiesLevel V: Nonexperimental/observational studyLevel IV: Systematic review of nonexperimental (observational) studies Level III: Nonrandomized trial (quasi-experiment) Level II: Randomized controlled trial (RCT)Level I: Systematic review/meta-analysis of RCTsVIIVIVIVIIIIII Level VIII: Nonresearch source (e.g., internal evidence, expert opinion)Level VII: Qualitative study/descriptive studyLevel VI: Systematic review/metasynthesis of qualitative studiesLevel V: Nonexperimental/observational studyLevel IV: Systematic review of nonexperimental (observational) studies Level III: Nonrandomized trial (quasi-experiment) Level II: Randomized controlled trial (RCT)Level I: Systematic review/meta-analysis of RCTs
From Polit and Beck (2021, Figure 2.2, p. 29)
The Point

"Figure 2.2 [in context of book] shows our eight-level evidence hierarchy for Therapy/intervention questions. This hierarchy ranks sources of evidence with respect the readiness of an intervention to be put to use in practice" (Polit & Beck, 2021, p. 28). Levels are ranked on risk of bias - level one being the least bias, level eight being the most biased. There are several types of levels of evidence scales designed for answering different questions. "An evidence hierarchy for Prognosis questions, for example, is different from the hierarchy for Therapy questions" (p. 29).

Advantages of Levels of Evidence Scales

"Through controls imposed by manipulation, comparison, and randomization, alternative explanations can be discredited. It is because of this strength that meta-analyses of RCTs, which integrate evidence from multiple experiments, are at the pinnacle of the evidence hierarchies for Therapy questions" (p. 188).

"Tip: Traditional evidence hierarchies or level of evidence scales (e.g., Figure 2.2), rank evidence sources almost exclusively based on the risk of internal validity threats" (p. 217).

Systematic reviews can provide researchers with knowledge that prior evidence shows. This can help clarify established efficacy of a treatment without unnecessary and thus unethical research. Greenhalgh (2019) illustrates this citing Dean Fergusson and colleagues (2005) systematic review on a clinical surgical topic (p. 128).

Limits of Levels of Evidence Scales

Regarding the importance of real-world clinical practice settings, and the conflicting tradeoffs between internal and external validity, Polit and Beck (2021) write, "the first (and most prevalent) approach is to emphasize one and sacrifice another. Most often, it is external validity that is sacrificed. For example, external validity is not even considered in ranking evidence in level of evidence scales" (p. 221). ... From an EBP perspective, it is important to remember that drawing inferences about causal relationships relies not only on how high up on the evidence hierarchy a study is (Figure 2.2), but also, for any given level of the hierarchy, how successful the researcher was in managing study validity and balancing competing validity demands" (p. 222).

Polit and Beck note Levin (2014) that an evidence hierarchy "is not meant to provide a quality rating for evidence retrieved in the search for an answer" (p. 6), and as the Oxford Center for Evidence-Based Medicine concurs that evidence scales are, 'NOT intended to provide you with a definitive judgment about the quality of the evidence. There will inevitably be cases where "lower-level" evidence...will provide stronger than a "higher level" study (Howick et al., 2011, p.2)'" (p. 30).

Polit and Beck (2021) further explain the difference between levels of evidence and quality of evidence related to appraisal: "The first appraisal issue is the extent to which the findings in a research report are valid. That is, were the study methods sufficiently rigorous that the evidence has a low risk of bias? Melnyk and Fineout-Overholt (2019) propose the following formula:

Level of evidence (e.g., Figure 2.2) + Quality of evidence = Strength of evidence.

Thus, in coming to a conclusion about the quality of the evidence, it is insufficient to simply 'level' the evidence using an LOE scale–it must also be appraised" (p. 36). Greenhalgh (2019) likewise acknowledges, "on the negative side, systematic reviews can replicate and magnify flaws in the original studies (e.g. if all primary studies considered a drug at sub-therapeutic dose, the overall - misleading - conclusion may be that the drug has 'no effect')" (p. 19).

The 6S Model of Levels of Evidence

Level 6Level 5Level 4Level 3Level 2Level 1Original articles published in journalsBrief overviews of single studies, with commentaryBrief overviews of systematic reviews, with commentaryEvidence-based clinicalpractice guidelines orclinical pathwaysComputerized clinical information systems with automatic decision supports Systematic reviews or meta-analysesSynopsis of StudiesSynthesesStudiesSynopsis of SynthesesSummariesSystems SystemsLevel 1Computerized clinical information systems with automatic decision supports SummariesLevel 2Evidence-based clinical practice guidelines or clinical pathways Synopsis of SynthesesLevel 3Brief overviews of system-atic reviews, with commentaryLevel 4Systematic reviews ormeta-analyses SythesesLevel 5Brief overviews of singlestudies, with commentaryLevel 6Original articles published in journals Synopsis of StudiesStudies
Modified from DiCenso, Bayley, & Haynes (2009)
The Point

"The 6S hierarchy does not imply a gradient of evidence in terms of quality, but rather in terms of ease in retrieving relevant evidence to address a clinical question. At all levels, the evidence should be assessed for quality and relevance" (Polit & Beck, 2021, p. 24, Tip box).

The 6S Pyramid proposes a structure of quantitative evidence where articles that include pre-appraised and pre-synthesized studies are located at the top of the hierarchy (McMaster U., n.d.).

It can help to consider the level of evidence that a document represents, for example, a scientific article that summarizes and analyses many similar articles may provide more insight than the conclusion of a single research article. This is not to say that summaries can not be flawed, nor does it suggest that rare case studies should be ignored. The aim of health research is the well-being of all people, therefore it is important to use current evidence in light of patient preferences negotiated with clinical expertise.

Other Gradings in Levels of Evidence

Schub & Walsh (2017) explain that:

While it is accepted that the strongest evidence is derived from meta-analyses, various evidence grading systems exist. for example:

The Johns Hopkins Nursing Evidence-Based Practice model ranks evidence from level I to level V, as follows (Seben et al., 2010):
  • Level I: Meta-analysis of randomized clinical trials (RCTs); experimental studies; RCTs
  • Level II: Quasi-experimental studies
  • Level III: Non-experimental or qualitative studies
  • Level IV: Opinions of nationally recognized experts based on research evidence or an expert consensus panel
  • Level V: Opinions of individual experts based on non-research evidence (e.g., case studies, literature reviews, organizational experience, and personal experience)
The American Association of Critical-Care Nurses (AACN) evidence level system, updated in 2009, ranks evidence as follows (Armola et al., 2009):
  • Level A: Meta-analysis of multiple controlled studies or meta-synthesis of qualitative studies with results that consistently support a specific action, intervention, or treatment
  • Level B: Well-designed, controlled randomized or non-randomized studies with results that consistently support a specific action, intervention, or treatment
  • Level C: Qualitative, descriptive, or correlational studies, integrative or systematic reviews, or RCTs with inconsistent results
  • Level D: Peer-reviewed professional organizational standards, with clinical studies to support recommendations
  • Level E: Theory-based evidence from expert opinion or multiple case reports
  • Level M: Manufacturers’ recommendations (2017)

EBM Pyramid and EBM Page Generator

Click on the pyramid text or scroll to view resources described by each level of evidence. Page source: EBM Pyramid and EBM Page Generator for Evidence-Based Mental Health Resources by Jan Glover, David Izzo, Karen Odato and Lei Wang.

Unfiltered are resources that are primary sources describing original research. Randomized controlled trials, cohort studies, case-controlled studies, and case series/reports are considered unfiltered information.

Filtered are resources that are secondary sources which summarize and analyze the available evidence. They evaluate the quality of individual studies and often provide recommendations for practice. Systematic reviews, critically-appraised topics, and critically-appraised individual articles are considered filtered information.


Armola, R. R., Bourgault, A. M., Halm, M. A., Board, R. M., Bucher, L., Harrington, L., ... Medina, J. (2009). AACN levels of evidence. What's new? Critical Care Nurse, 29(4), 70-73. doi:10.4037/ccn2009969

DiCenso, A., Bayley, L., & Haynes, R. B. (2009). Accessing pre-appraised evidence: Fine-tuning the 5S model into a 6S model. BMJ Evidence-Based Nursing, 12(4)

Fergusson, D., Glass, K. C., Hutton, B., & Shapiro, S. (2005). Randomized controlled trials of Aprotinin in cardiac surgery: Could clinical equipoise have stopped the bleeding?. Clinical Trials, 2(3), 218-232.

Glover, J., Izzo, D., Odato, K. & Wang, L. (2008). Evidence-based mental health resources. EBM Pyramid and EBM Page Generator. Copyright 2008. All Rights Reserved. Retrieved April 28, 2020 from Note. Document removed from host. Old link used with the WayBack Machine of the Internet Archive to retrieve the original webpage on 2/10/21

Greenhalgh, T. (2019). How to read a paper: The basics of evidence-based medicine and healthcare. (Sixth ed.). Wiley Blackwell.

Haynes, R. B. (2001). Of studies, syntheses, synopses, and systems: The “4S” evolution of services for finding current best evidence. BMJ Evidence-Based Medicine, 6(2), 36-38.

Haynes, R. B. (2006). Of studies, syntheses, synopses, summaries, and systems: the “5S” evolution of information services for evidence-based healthcare decisions. BMJ Evidence-Based Medicine, 11(6), 162-164.

McMaster University (n.d.). 6S Search Pyramid Tool

Polit, D., & Beck, C. (2019). Nursing research: Generating and assessing evidence for nursing practice. Wolters Kluwer Health.

Schub, E., Walsh, K. & Pravikoff D. (Ed.) (2017). Evidence-based nursing practice: Implementing [Skill Set]. Nursing Reference Center Plus

Seben, S., March, K. S., & Pugh, L. C. (2010). Evidence-based practice: The forum approach. American Nurse Today, 5(11), 32-34.

What are Systematic Reviews?


Standards & Guides

Cochrane, 2016

Read More


Cochrane [Username]. (2016, Jan 27). What are systematic reviews? YouTube.

Davies, A. (2019). Carrying out systematic literature reviews: An introduction. British Journal of Nursing, 28(15), 1008–1014.

Greenhalgh, T. (2019). Papers that summarize other papers (systematic reviews and meta-analyses). In How to read a Paper : The basics of evidence-based medicine and healthcare. (Sixth ed., pp. 117-136). Wiley Blackwell.

Holly, C. (2017). Systematic review. In J. Fitzpatrick (Ed.), Encyclopedia of nursing research (4th ed.). Springer Publishing Company. Credo Reference.

Zhang, J., Han, L., Shields, L., Tian, J., & Wang, J. (2019). A PRISMA assessment of the reporting quality of systematic reviews of nursing published in the Cochrane Library and paper-based journals. Medicine, 98(49), e18099.