"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).
"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).
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.
"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.
Schub & Walsh 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):The American Association of Critical-Care Nurses (AACN) evidence level system, updated in 2009, ranks evidence as follows (Armola et al., 2009):
- 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)
- 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)
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) https://ebn.bmj.com/content/12/4/99.2.short
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 https://web.archive.org/web/20200219181415/http://www.dartmouth.edu/~biomed/resources.htmld/guides/ebm_psych_resources.html 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 http://www.dartmouth.edu/~biomed/resources.htmld/guides/ebm_psych_resources.html
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 https://www.nccmt.ca/capacity-development/6s-search-pyramid
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.
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.
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