Evidence synthesis

Evidence synthesis refers to analyzing studies within the context of knowledge about a subject. It is to fuse together different types of information about a particular topic. Examples could the use of the evidence in medicine, policy making, education, criminal justice, food safety, conservation, and other fields. Use of evidence from multiple studies helps to give a better understanding of an issue. Evidence synthesis is used by researchers, individuals, and organizations. Many working in the field of evidence synthesis view it as a type of research in its own right. Review articles typically have high citation rates, and review journal content is influential. Evidence synthesis and traditional reviews of literature on a topic have much in common, although evidence synthesis is more time-intensive and typically requires a research team.

Evaluating evidence synthesis can be useful when the method used is transparent. This includes information about how the researchers searched for the studies, if they were consistent in deciding which to include and exclude, how they decided which results to highlight, how useful the review of evidence is, how clear is the way the research team reached its conclusions, and how reliable, recent, and trustworthy the studies include are. Quality evidence synthesis can help professionals and the public gain a better understanding of a topic and make better decisions.

Background

The scientific method has developed over thousands of years. Philosopher Francis Bacon is credited with shaping modern ideas about the scientific method in the late sixteenth and early seventeenth centuries. His ideas make it clear that science is more than data. It requires analysis of the information. Researchers took this idea further by expanding their analysis using data from other studies and experiments.

By the nineteenth century, major figures in science advocated for analysis that made clear the relation of new information to what had previously been discovered. This was a laborious process that required researchers to pore over the text of multiple studies to find quality data. The science of evidence synthesis really began to develop in earnest during the twentieth century. However, thorough searching of studies remained extremely difficult until the late twentieth century. The Internet and computerized databases now allow users to search for key terms and acquire complete reports of studies that were conducted during earlier decades and centuries.

During the late twentieth century, many researchers used evidence synthesis to evaluate medical treatments. Their results often suggested that further research was needed, although in some cases scrutiny of the combined data was clear. For example, one analysis combined multiple studies of the routine use of radiation following mastectomy for early breast cancer. The researchers concluded that although radiation clearly reduced the chance of cancer recurrence, it also posed significant risk to the patients. They suggested that survival rates would be higher if routine radiation treatment were stopped.

Having established the benefits of evidence synthesis, multiple organizations were created to produce these reviews. Among these is the Cochrane Collective, which is the largest single producer of reviews on health and medical topics. As an indication of the volume of studies housed by the Cochrane Library, by the mid-2020s there were over 7,000 Plain Language Summaries (PLS), covering more than thirty topics, of research findings included in Cochrane Reviews. Other organizations such as the Campbell Collaboration focus on development, education, and social welfare.

Overview

Reasons to use evidence synthesis include organizing a collection of evidence, evaluating evidence quality, minimizing bias, comparing and contrasting similar studies, combining findings to improve the power of statistics, making the evidence more accessible, and to design better future studies.

Evidence synthesis begins with a question and commences as research and analysis of existing data on the topic. It is used to gather evidence for actions and to make policies. It is also used to identify areas of research that remain unexplored. Commonly used types of evidence synthesis include systematic review, a scoping review or evidence map, rapid review, umbrella review, and meta-analysis. The latter may stand alone or be included in a systematic review. A literature or narrative review, which typically casts a wide net for data, may also be classified as evidence synthesis.

Systematic reviewthe most common type of evidence synthesiscollects and categorizes data on a broad question. It is thorough and time-consuming, typically taking months or years.

A scoping review or an evidence map also views and analyzes data on broad questions. Unlike a systematic review, it does not synthesize results, and may take even longer to complete. It maps what has been studied and identifies gaps in the research as well as areas where evidence synthesis should be performed.

Rapid reviews use the methods of systematic review but do so within a shorter time frame. This may require limits on the review that can lead to bias, but a rapid review has the benefit of informing decisions more quickly.

An umbrella review is a review of other systematic reviews on a given topic. The question that launches an umbrella review may be even more broad. It can be useful when systematic reviews have found multiple outcomes.

Meta-analysis refers to analyzing analyses. The method arose during the 1970s when leaders in the research field advocated for better synthesis of study results through statistical methods. For example, many studies of a topic may individually have included very few subjects, but combining them may provide a large pool of data. However, even when the studies examine the same topic, the subjects used may not be comparable, or the way data was collected may be incompatible with the reason a researcher wants to combine them.

Various journals and fields follow specific reporting standards for evidence synthesis. These may establish minimum sets of items that may be reported in reviews. Among these are the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and standards for protocol development (PRISMA-P), scoping reviews and evidence maps (PRISMA-ScR), and individual participant data (PRISMA-IPD). Additionally, the Enhancing the Quality and Transparency of Health Research (EQUATOR) Network was established to improve accuracy and transparency in international systematic reviews.

Qualitative evidence synthesis (QES) process that has evolvedspecific methodologies used to take systematic reviews of research. QES are employed mainly in health, social, care, and education fields. A QES will often occur after an initial study has identified potential areas for further research. They will also attempt to add to the understanding of a subject beyond a discussion of an effective solution that applies only to a specific circumstance. A QES will later be used for decision-making in positions such as policymaking and guideline development. There are now over thirty established methods for conducting a qualitative evidence synthesis.

Bibliography

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