Which type of review is considered the most rigorous?

Timeframe: ≤ 5 weeks (varies)   *Varies beyond the type of review. Depends on many factors such as but not limited to: resources available, the quantity and quality of the literature, and the expertise or experience of reviewers" (Grant et al. 2009)

Question: Narrow question, may use PICO 

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Sources and searches: Sources are limited due to time constraints of searching, however still uses transparent and reproducible search methods.

There are many types of literature review. The choice of a specific type depends on your research approach and design. The following types of literature review are the most popular in business studies:

Narrative literature review, also referred to as traditional literature review, critiques literature and summarizes the body of a literature. Narrative review also draws conclusions about the topic and identifies gaps or inconsistencies in a body of knowledge. You need to have a sufficiently focused research question to conduct a narrative literature review

Systematic literature review requires more rigorous and well-defined approach compared to most other types of literature review. Systematic literature review is comprehensive and details the timeframe within which the literature was selected. Systematic literature review can be divided into two categories: meta-analysis and meta-synthesis.

When you conduct meta-analysis you take findings from several studies on the same subject and analyze these using standardized statistical procedures. In meta-analysis patterns and relationships are detected and conclusions are drawn. Meta-analysis is associated with deductive research approach.

Meta-synthesis, on the other hand, is based on non-statistical techniques. This technique integrates, evaluates and interprets findings of multiple qualitative research studies. Meta-synthesis literature review is conducted usually when following inductive research approach.

Scoping literature review, as implied by its name is used to identify the scope or coverage of a body of literature on a given topic. It has been noted that “scoping reviews are useful for examining emerging evidence when it is still unclear what other, more specific questions can be posed and valuably addressed by a more precise systematic review.” The main difference between systematic and scoping types of literature review is that, systematic literature review is conducted to find answer to more specific research questions, whereas scoping literature review is conducted to explore more general research question.

Argumentative literature review, as the name implies, examines literature selectively in order to support or refute an argument, deeply imbedded assumption, or philosophical problem already established in the literature. It should be noted that a potential for bias is a major shortcoming associated with argumentative literature review.

Integrative literature review reviews, critiques, and synthesizes secondary data about research topic in an integrated way such that new frameworks and perspectives on the topic are generated. If your research does not involve primary data collection and data analysis, then using integrative literature review will be your only option.

Theoretical literature review focuses on a pool of theory that has accumulated in regard to an issue, concept, theory, phenomena. Theoretical literature reviews play an instrumental role in establishing what theories already exist, the relationships between them, to what degree existing theories have been investigated, and to develop new hypotheses to be tested.

At the earlier parts of the literature review chapter, you need to specify the type of your literature review your chose and justify your choice. Your choice of a specific type of literature review should be based upon your research area, research problem and research methods.  Also, you can briefly discuss other most popular types of literature review mentioned above, to illustrate your awareness of them.

Munn, A. et. al. (2018) “Systematic review or scoping review? Guidance for authors when choosing between a systematic or scoping review approach” BMC Medical Research Methodology

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.

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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.

Conclusion

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).

What is the most rigorous research evidence review?

Which review is considered the highest level of research evidence?

Are systematic reviews specific and rigorous?

Why are rigorous methods of systematic review important?