What are Evidence Syntheses?

What are Evidence Syntheses?

According to the Royal Society, 'evidence synthesis' refers to the process of bringing together information from a range of sources and disciplines to inform debates and decisions on specific issues. They generally include a methodical and comprehensive literature synthesis focused on a well-formulated research question. Their aim is to identify and synthesize all of the scholarly research on a particular topic, including both published and unpublished studies. Evidence syntheses are conducted in an unbiased, reproducible way to provide evidence for practice and policy-making, as well as to identify gaps in the research. Evidence syntheses may also include a meta-analysis, a more quantitative process of synthesizing and visualizing data retrieved from various studies. 

Evidence syntheses are much more time-intensive than traditional literature reviews and require a multi-person research team. See this PredicTER tool to get a sense of a systematic review timeline (one type of evidence synthesis). Before embarking on an evidence synthesis, it's important to clearly identify your reasons for conducting one. For a list of types of evidence synthesis projects, see the next tab.

How Does a Traditional Literature Review Differ From an Evidence Synthesis?

How does a Systematic Review Differ from a Traditional Literature Review?

One commonly used form of evidence synthesis is a systematic review.  This table compares a traditional literature review with a systematic review.

 

Traditional Literature Review

Systematic Review

Review Question/Topic

Topics may be broad in scope; the goal of the review may be to place one's own research within the existing body of knowledge, or to gather information that supports a particular viewpoint.

Starts with a well-defined research question to be answered by the review. Reviews are conducted with the aim of finding all existing evidence in an unbiased, transparent, and reproducible way.

Searching for Studies

Searches may be ad hoc and based on what the author is already familiar with. Searches are not exhaustive or fully comprehensive.

Attempts are made to find all existing published and unpublished literature on the research question. The process is well-documented and reported.

Study Selection

Often lack clear reasons for why studies were included or excluded from the review.

Reasons for including or excluding studies are explicit and informed by the research question.

Assessing the Quality of Included Studies

Often do not consider study quality or potential biases in study design.

Systematically assesses risk of bias of individual studies and overall quality of the evidence, including sources of heterogeneity between study results.

Synthesis of Existing Research

Conclusions are more qualitative and may not be based on study quality.

Bases conclusion on quality of the studies and provide recommendations for practice or to address knowledge gaps.

Video: Reproducibility and transparent methods (Video 3:25)

Reporting Standards

Reporting Standards

There are some reporting standards for evidence syntheses. These can serve as guidelines for protocol and manuscript preparation and journals may require that these standards are followed for the review type that is being employed (e.g. systematic review, scoping review, etc).​

Video: Guidelines and reporting standards

PRISMA Flow Diagram

PRISMA Flow Diagram

The PRISMA flow diagram depicts the flow of information through the different phases of an evidence synthesis. It maps the search (number of records identified), screening (number of records included and excluded), and selection (reasons for exclusion). Many evidence syntheses include a PRISMA flow diagram in the published manuscript.

See below for resources to help you generate your own PRISMA flow diagram.