About Systematic Reviews

What Level of Evidence Is a Systematic Review?

Healthcare decisions involved in medical practice and public health policymaking must be informed by the best available research evidence. This evidence comes from good systematic reviews which are a state-of-the-art synthesis of all the current evidence to answer a specific research question. Systematic reviews aim to identify, evaluate, and summarize the findings of all relevant individual studies to answer a health-related research question, thereby making all the available evidence more accessible to decision-makers and practitioners. There are different types of systematic reviews in research developed to address each type of research question. One such review type is a scoping review. For guidance on conducting systematic scoping reviews, you can learn more at the previous link.

Given the explosion of medical literature, and the need to use high-quality evidence to inform healthcare decision-making, a hierarchical system was developed to classify available evidence. This hierarchy, known as the levels of evidence, is the cornerstone of evidence-based medicine. In this article, we will look at levels of evidence in further detail, and see where systematic reviews stand in this hierarchy.

What Are The Levels Of Evidence?

The Levels of Evidence, also known as the hierarchy of evidence, is a heuristic method used to rank the relative strengths of the results obtained from scientific research. Due to the insurmountable amount of available research, the evidence-based medicine movement organizes and assesses this huge volume and diversity of evidence with ‘evidence hierarchies’. In 2014, Jacob Stegenga defined an evidence hierarchy as ‘a rank ordering of methods according to the potential for that method to suffer from systematic bias’ [1]. The rank is usually determined by one or more parameters of the study design. The elements that are ordered in evidence hierarchies are usually different kinds of methods, and the property on which the ordering is based is taken to be the internal validity of a method relative to the hypotheses regarding the efficacy of tested medical intervention [2]. Therefore, at the top of the hierarchy are studies with the highest internal validity or lowest risk of bias relative to the tested medical intervention’s hypothesized efficacy.


The need for developing a hierarchical system for the classification of the available evidence was pointed out by Archibald Cochrane with the publication of ‘Effectiveness and Efficiency’ in 1972, in which he argued that decisions about medical treatment should be based on a systematic review of clinical evidence[3]. In 1979, the Canadian Task Force on Periodic Health Examination published the first-ever ranking of medical evidence, proposing four quality levels [4]. These levels were used to assign an alphabetical grade to the strength of recommendations or interventions. The levels of evidence were further described and expanded by Sackett in 1989. Both systems placed Randomized controlled trials (RCT) at the highest level and case series or expert opinions at the lowest level, based on the probability of bias.

Gordon Guyatt, the Canadian physician, who coined the term ‘evidence-based medicine’ in 1991, proposed another approach for classifying the strengths of recommendations. In ‘Users Guide to medical literature’, Guyatt expanded the existing categorization to account for new systematic procedures for combining results from different studies [5]. Referencing Guyatts paper, Trisha Greenhalgh summarized the revised hierarchy as follows [6],

  1. Systematic reviews and Meta-analyses
  2. Randomized controlled trials with definitive results (confidence intervals that do not overlap the threshold of clinically significant effect)
  3. Randomized controlled trials with non-definitive results (a point estimate that suggests a clinically significant effect but with confidence intervals overlapping the threshold for this effect)
  4. Cohort studies
  5. Case-control studies
  6. Cross-sectional surveys
  7. Case reports

It was proposed that modifications to the levels of evidence are required since some studies answer a particular question better than others. Therefore, different variations of levels of evidence classification exist based on the type of clinical question asked. The levels of evidence also take into account the quality of data.

Where Do Systematic Reviews Stand In the Level of Evidence Hierarchy?

In most evidence hierarchies, well-conducted systematic reviews and meta-analyses are at the top. As such, in the hierarchy of evidence, systematic reviews including meta-analysis of methodologically sound RCTs with consistent results, are considered the highest level of evidence [5]. This is due to the fact that systematic reviews not only offer the benefit of collating all the available evidence from a variety of sources but also critically appraise the quality of the evidence collected. The level of evidence attributed to a systematic review with meta-analysis also owes to its methodological design which is focused on the minimization of bias. Bias (of which there are many types) can confound the outcomes of a study such that it may over or underestimate the true treatment effect. Therefore, systematic reviews of randomized control trials which are designed specifically to minimize bias from confounding factors have become the highest level of evidence. However, the position of systematic reviews at the top is not absolute. For example,

  • The process of conducting a systematic review is rigorous and it is estimated that it takes between 6 and 18 months to complete the procedure depending on the topic. Therefore, the results generated by a systematic review could be superseded by new evidence.
  • The results of a large, well conducted randomized controlled trial (RCT) could be more convincing than a systematic review of smaller inefficient RCTs.


Evidence-based medicine relies on ‘the best available evidence’, and to fully understand this one needs to have a clear knowledge of the hierarchy of evidence and how it can be used to formulate a grade of recommendation. It is critical to establish which evidence is the most authoritative for a particular subject. The levels of evidence provide a guide and researchers need to be careful while interpreting their results. They specify a hierarchical order for different researches based on their internal validity (What Are the Levels of Evidence? – Center for Evidence-Based Management).


  1. Stegenga J (October 2014). “Down with the hierarchies”. Topoi. 33 (2): 313–22. doi:10.1007/s11245-013-9189-4. S2CID 109929514.
  2. Borgerson K. Valuing evidence: bias and the evidence hierarchy of evidence-based medicine. Perspect Biol Med. 2009 Spring;52(2):218-33. doi: 10.1353/pbm.0.0086. PMID: 19395821.
  3. Stavrou, A.; Challoumas, D.; Dimitrakakis, G. “Archibald Cochrane (1909–1988): the father of evidence-based medicine.” Interactive Cardiovascular and Thoracic Surgery, 18(1) (2014): 121-124.
  4. Spitzer, W. et al. The periodic health examination. Canadian Task Force on the Periodic Health Examination. (1979). Canadian Medical Association journal, 121(9), 1193–1254.
  5. Guyatt, G. H.; Sackett, D. L.; Sinclair, J. C.; Hayward, R.; Cook, D. J.; Cook, R. J. “Users’ guides to the medical literature IX. A method for grading health care recommendations.” JAMA, 274 (22) (1995): 1800-1804.
  6. Greenhalgh T. How to read a paper. Getting your bearings (deciding what the paper is about). BMJ. 1997;315(7102):243-246. doi:10.1136/bmj.315.7102.243
  7. What Are the Levels of Evidence? – Center for Evidence Based Management. cebma.org/faq/what-are-the-levels-of-evidence/.

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