"Evidence syntheses combine data from multiple sources, most commonly from existing research studies, to provide an overall summary of current knowledge. 

Evidence syntheses should be based on systematic processes to identify and collate relevant evidence that meets pre-specified eligibility criteria and to minimise the influence of bias. Combining individual study results and considering differences can reduce uncertainty and help make sense of conflicting study findings.

• Systematic reviews - aim to identify and formally evaluate all the studies addressing a clearly defined research question using methods that are set out in advance
• Rapid reviews - when an answer is needed quickly, researchers can use streamlined methods to produce a rapid review
• Scoping reviews or production of an evidence map – these can be carried out if we want to know what studies exist without looking in detail at their findings
• Statistical analysis  - evidence syntheses may include a statistical analysis that combines individual study results. This is called a meta-analysis or a network meta-analysis when three or more comparisons are being made within a single analysis
• Decision analytic modeling - is a means of estimating the costs, outcomes and cost-effectiveness of different interventions and programmes

Whatever kind of evidence synthesis is conducted, appropriate, rigorous and transparent methods must be used so that the conclusions can be trusted."

Evidence Synthesis, National Institute for Health and Care Research (UK)

"Evidence synthesis, sometimes called “systematic reviews”, involves combining information from multiple studies investigating the same topic to comprehensively understand their findings. This helps determine how effective a particular treatment or drug is, or how people have experienced a particular health condition or treatment. By using evidence synthesis effectively, policymakers, healthcare institutions, clinicians, researchers, and the public can make more informed decisions about health and healthcare. " 

Evidence Synthesis - What is it and why do we need it?, Cochrane