Evidence Hierarchy in Evidence-Based Medicine

Evidence-based medicine, also known as EBM, relies on evidence to make a decision. EBM engages the hierarchy of evidence to evaluate and assess the evidence that it has; five levels will be examined below.

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Case Report

Case reports and case studies are considered to be the weakest level of the hierarchy. They are usually descriptive and present the reader with a set of factors that have contributed to the development of illnesses (Hoe & Hoare, 2012). No control group is involved: a case report only includes information about a disease. It can also be a qualitative study without any numerical data.

Empirical Level

At this level, specific studies that include control groups are used. They are called case-control studies; during these studies, people with a disease are compared to people without it (the controls) (Hoe & Hoare, 2012). To understand what differences there are between these groups, scientist collects data from both groups and compare them. It is important to notice that these studies do not use randomization, but their results are usually published in peer-reviewed journals and can be trusted, although they might lack specific interventions.

Randomized Controlled Trial (RCT)

Randomized controlled trials are the next level of the hierarchy; these trials are designed and conducted in a way that makes them unbiased. Moreover, these trials also “have less risk of systematic errors” (Burns, Rohrich, & Chung, 2012, p. 308). Their main advantage is that they usually cannot be biased by the author’s experience, as it might happen in a case report. However, it should be noted that if an RCT is not designed and conducted properly, it might lead to biases and systematic errors (Burns et al., 2012; Pooler, 2014). To ensure that an RCT in unbiased, the following items can be used: “randomization, blinding, a description of the randomization and blinding process, description of the number of subjects who withdrew or drop out of the study” and others (Burns et al., 2012, p. 306). If randomization was not conducted properly, there is a big chance that the study will be of low quality.

Systematic Literature Review

Systematic literature reviews examine, discuss, and analyze evidence from different sources and studies, usually RCTs. If published RCTs that regard a specific issue or illness are unavailable, systematic literature reviews may examine observational studies or other types of qualitative studies. Thereby, the evidence is formed basing on the results from both RCTs and qualitative studies. These literature reviews also assess the quality of the studies included. The results indicate whether an intervention was successful or not and what changes this intervention might need (Hoe & Hoare, 2012). Systematic reviews approach literature on a topic and compare the information from the given articles and studies. Systematic reviews can include a meta-analysis, but meta-analyses approach the evaluation of information differently.


The purpose of a meta-analysis is to compare and assess numerical data from several studies; the use of statistical tools and programs is encouraged when conducting a meta-analysis; otherwise, it is often impossible to compare the sets of numerical data. Meta-analyses extract data from RCTs to ensure that there is little to no biases in the studies; then, “a weighted average [is] calculated” (Hoe & Hoare, 2012, 55). Thus, meta-analyses provide more detailed, statistically correct, and unbiased information than systematic reviews. However, as meta-analyses rely on other studies’ results, there is a chance that if one of the studies is biased, a meta-analysis will be biased too (Tuck et al., 2014). Evidence gap in RCTs can also lead to false conduction of a meta-analysis.


Burns, P. B., Rohrich, R. J., & Chung, K. C. (2012). The levels of evidence and their role in evidence-based medicine. Plastic and Reconstructive Surgery, 128(1), 305-310.

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Hoe, J., & Hoare, Z. (2012). Understanding quantitative research: Part 1. Nursing Standard, 27(15), 52-57.

Pooler, A. (2014). An introduction to evidence-based practice in nursing & healthcare. London, UK: Routledge.

Tuck, S. L., Winqvist, C., Mota, F., Ahnström, J., Turnbull, L. A., & Bengtsson, J. (2014). Land‐use intensity and the effects of organic farming on biodiversity: A hierarchical meta‐analysis. Journal of Applied Ecology, 51(3), 746-755.

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