The 6S Model is designed to work from the top down, starting with Systems - also referred to as computerized decision support systems (CDSSs). DiCenso et al. describes that, “an evidence-based clinical information system integrates and concisely summarizes all relevant and important research evidence about a clinical problem, is updated as new research evidence becomes available, and automatically links (through an electronic medical record) a specific patient’s circumstances to the relevant information” (2009). Systematic reviews lead up to this type of bio-available level of evidence.
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L ev el VIII: Nonresearch source (e.g., internal evidence, expert opinion)
L ev el VII: Qualitative study/descriptive study
L ev el VI: Systematic review/metasynthesis of qualitative studies
L ev el V : Nonexperimental/observational study
L ev el I V : Systematic review of nonexperimental (observational) studies
L ev el III: Nonrandomized trial (quasi-experiment)
L ev el II: Randomized controlled trial (RCT)
L ev el I: Systematic review/meta-analysis of RC T s
L ev el VIII: Nonresearch source (e.g., internal evidence, expert opinion)
L ev el VII: Qualitative study/descriptive study
L ev el VI: Systematic review/metasynthesis of qualitative studies
L ev el V : Nonexperimental/observational study
L ev el I V : Systematic review of nonexperimental (observational) studies
L ev el III: Nonrandomized trial (quasi-experiment)
L ev el II: Randomized controlled trial (RCT)
L ev el I: Systematic review/meta-analysis of RC T s
From Polit and Beck (2021, Figure 2.2, p. 29)
"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 .
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).