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.

History

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.