Evidence synthesis is the process of retrieving, evaluating and summarising the findings of all relevant studies on a certain subject area. Ideally, a systematic review is conducted to identify all the relevant available studies to support the evidence synthesis. For more information about systematic reviewing, see the Cochrane handbook for systematic reviews of interventions. (a description of a systematic literature review in the context of exploring and identifying drivers of effectiveness is found here).
Meta-analyses may then be used to combine the estimates from the individual studies identified.
Network meta-analysis (NMA) is an extension of the standard, pairwise meta-analysis, and can be used to synthesise results from studies that compare multiple competing interventions for the same condition.
For more information about evidence synthesis and network meta-analysis see here.
Including RWD in evidence synthesis
Meta-analysis and NMA are usually limited to the synthesis of evidence from randomised controlled trials (RCTs) because they are considered to be the most reliable source of information on relative treatment effects. However, there is a growing interest in the medical community in incorporating evidence from non-randomised studies (NRSs), patient registries and other real-world data (RWD).
This strategy is particularly appealing when there are few RCTs to answer a specific research question. It may also be useful when the available RCTs do not align with the target population, prescription strategies and/or primary outcomes of the research question (i.e. when there is an efficacy-effectiveness gap, see a definition here).
Including RWD may be also be helpful to connect disconnected networks of interventions (i.e. if trials comparing interventions are not available) or to supplement existing RCT evidence when the results are conflicting or evidence is limited.
For more information about incorporating RWD into an NMA see here.