Evidence Synthesis

What is involved in data synthesis?

During the data synthesis step, the main findings of the review need to be presented.

Data synthesis:

• Data from included studies is analysed and synthesised to answer the review question.
• A common method of synthesis is meta-analysis, a statistical approach that brings together results from multiple studies where the data is sufficiently similar (homogeneous).
• If the studies are heterogeneous, they cannot be synthesised into a meta-analysis. In this case, you would use narrative or descriptive to describe the results from each study.
• Qualitative data can be synthesised using meta-synthesis.

Meta-Analysis

If the studies you have included in your evidence synthesis are sufficiently similar, or in other words, homogenous, you can synthesise the data from these studies using a process called "meta-analysis.” As the name suggests, a meta-analysis uses a statistical approach to bring together results from multiple studies.

Narrative/Descriptive synthesis

If the studies you have included in your evidence synthesis are not similar (e.g. you have included different research designs due to diversity in the evidence base), then a meta-analysis is not possible. In this instance, you can synthesise the data from these studies using a process called “narrative or descriptive synthesis”.

A word of caution here: while the process underpinning meta-analysis is well established and standardised, the process underpinning narrative or descriptive synthesis is subjective and there is no one standard process for undertaking this.

Meta-Synthesis

In recent times, evidence syntheses of qualitative research are gaining popularity. Data synthesis in these studies may be termed as “meta-synthesis”. As with narrative or descriptive synthesis, there are a myriad of approaches to meta-synthesis.

Regardless of whether an evidence synthesis presents qualitative or quantitative information, reporting using the PRISMA flow diagram is recommended. The PRISMA website and its many adaptations can be very helpful in understanding components of systematic reviews, meta-analyses, and related evidence synthesis methods.