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Systematic reviews (SRs) are the embodiment of the evidence-based approaches, and they have reformed clinical decision-making in almost all therapy areas. The approach of SRs was essentially developed to fulfil the need of the medical practitioners to obtain precise and consistent information about the efficacy and safety of a clinical intervention, diagnostic procedure, or a prognostic marker from a pool of evidence, which is apparently full of contradiction, heterogeneity and bias.(1) Although SRs and meta-analyses are robust and detail-oriented, they’re both resource intense, with a limited scope of outcomes.(2) In order to cater to a range of needs from the stakeholders, the SR approach has branched within the realm of evidence synthesis. For instance, rapid reviews provide for more urgent deadlines but may not follow all the methods of an SR,(3) scoping reviews include larger bodies of evidence, not requiring a detailed synthesis,(4) and realist reviews focus on the assessment of the functions of complex interventions, often comprising of evidence excluded from classic SRs.(5, 6)

Given the resource intense nature of the SRs, it is essential to recognize the most informative research questions in order to maximize their value and efficiency in clinical and regulatory decision-making. It can be inefficient to invest resources in SRs barely as a means of addressing specific research questions, if data available to answer those questions is lacking. Therefore, decision-makers need to monitor and understand the evidence base as a whole, so as to quickly determine the emerging trends or issues of potential concern. This can, in turn, facilitate the development of proactive research questions by relevant stakeholders for SRs to answer.(1)

Evidence mapping is a new approach for the evidence review process. This approach can potentially expedite evidence surveillance in a clear and reproducible manner, thus offering a broader understanding of the existing evidence base through interactive yields.(1) Evidence maps and evidence visualizations are systematic evidence synthesis approaches, which work by displaying visually the gaps in evidence or study characteristics, and, at times, summarize study quality or synthesized evidence from multiple studies. Such an interactive and visual representation provides a quick overview of the existing evidence base, thereby helping stakeholders and researchers to immediately understand research priorities.(7) For these reasons, evidence maps are excellent tools that help in guiding clinical investigators to set the agenda for future research.(8)

Being a rather new concept, there has been no uniform definition of, or methodology for conducting, evidence maps yet. Mainly, evidence maps are referred to as tools of systematic organisation and illustration of evidence base with the intent to characterize the breadth, depth and methodology of relevant evidence, identifying gaps.(9) Another definition of evidence map is “an approach to providing a visual representation and critical assessment of evidence landscape for a particular topic or question”.(10) A more recent definition is developed from the published evidence maps, which turned out to be a systematic search of a broad field identifying gaps in knowledge and the needs for future research.(6) The last one thus takes evidence maps to be a user-friendly representation of evidence bases visually in a figure or graph, a table or a searchable database.(8)

Nonetheless, due to the lack of a uniform definition, the stakeholders may not essentially know what to expect while warranting an evidence map or identifying existing maps. Moreover, lack of a repository for evidence maps makes them difficult to locate, thus making it less likely for authors to develop existing approaches further.(11)

Essentially, evidence maps are primarily prepared by the relevant stakeholders (researchers, policy-makers, funders, and, most importantly, patients) by identifying the most important clinical questions to their context, and researching on the body of evidence that is available already. Next, the quality of the available evidence is assessed and conveyed to stakeholders. The final step includes the visual depiction of the most relevant data elements to the stakeholder; for e.g., focusing on the size of the body of evidence, comparisons made versus those avoided, populations studied versus those avoided, and risk of bias, among other factors.(8) At the end of this process, the gaps in the available evidence in the context of the original research question starts to become apparent, and this can be used to plan further research activities.

In conclusion, evidence maps offer a robust and transparent methodological framework with which to assess the evidence landscape in a detailed manner, and aid clinical and regulatory decision-making. The broad scope of evidence maps, through efficient use of resources, can substantially streamline evidence synthesis by preventing unnecessary duplication of work. Additionally, future text mining and machine learning advancements will further possibly reduce the resource intensity of the methodology.(8)

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References

1. Wolffe TAM, Whaley P, Halsall C, et al. Systematic evidence maps as a novel tool to support evidence-based decision-making in chemicals policy and risk management. Environ Int. 2019; 130:104871.
2. Bastian H, Glasziou P, Chalmers I. Seventy-five trials and eleven systematic reviews a day: how will we ever keep up? PLoS Med. 2010; 7(9):e1000326.
3. Tricco AC, Antony J, Zarin W, et al. A scoping review of rapid review methods. BMC Med. 2015; 13(1):224.
4. Colquhoun HL, Levac D, O’Brien KK, et al. Scoping reviews: time for clarity in definition, methods, and reporting. J Clin Epidemiol. 2014; 67(12):1291–4.
5. Pawson R, Greenhalgh T, Harvey G, et al. Realist review–a new method of systematic review designed for complex policy interventions. J Health Serv Res Policy. 2005; 10 suppl 1:21–34.
6. Miake-Lye IM, Hempel S, Shanman R, et al. What is an evidence map? A systematic review of published evidence maps and their definitions, methods, and products. Syst Rev. 2016; 5:28.
7. Evidence Maps and Evidence Visualizations. Patient-centered Outcomes Research Institute. Available at: https://www.pcori.org/impact/evidence-maps-and-evidence-visualizations
8. Alahdab F, Murad MH. BMJ Evidence-Based Medicine. 2019.
9. Katz DL, Williams AL, Girard C, et al. The evidence base for complementary and alternative medicine: methods of Evidence Mapping with application to CAM. Altern Ther Health Med 2003; 9:22–30.
10. Bethan C, O’Leary PW, Kaiser MJ, et al. Evidence maps and evidence gaps: evidence review mapping as a method for collating and appraising evidence reviews to inform research and policy. Environmental Evidence 2017; 6.
11. Evidence and gap maps: A comparison of different approaches. Oslo, Norway: The Campbell Collaboration. Retrieved from: campbellcollaboration.org/ DOI: https://doi.org/10.4073/cmdp.2018.2