Data synthesis is the point at which the extracted data (results of individual research studies) are combined and then evaluated. This process of combining data from different studies is one of the critical differences between a systematic review and other types of literature reviews.
There are two commonly accepted methods of synthesis you can use when conducting a systematic review:
How the data gets synthesised is dependent on the type of data that has been extracted.
Quantitative systematic reviews will present the data in a statistical format. This approach is often referred to as a meta-analysis. Meta-analysis can be defined as a "statistical method that combines the results of a number of studies to calculate a single summary effect" (Munn et al., 2014) and is used to bring together data that is sufficiently similar (homogeneous).
A meta-analysis has a number of advantages over other methods such as having a greater sample size which leads to a more precise assessment of any effects and enables more variables be examined. Taking a statistical approach provides greater transparency and reduces the chance of bias or errors when drawing conclusions.
Please note that "while meta-analysis is preferred, it's not always possible, especially if the studies vary greatly, either in how they are conducted (different methodologies), what they are assessing (different interventions), whom they are performed on (different populations), or what their final result is. When such differences exist across studies, the studies are said to be heterogeneous" (Munn et al., 2014). In situations where the data is heterogeneous you would use a qualitative approach to describe the results from each study. This type of synthesis is considered more subjective.
References:
Munn, Z., Tufanaru, C., & Aromataris, E. (2014). JBI's systematic reviews: Data extraction and synthesis. AJN The American Journal of Nursing, 114(7). https://journals.lww.com/ajnonline/Fulltext/2014/07000/JBI_s_Systematic_Reviews__Data_Extraction_and.28.aspx
Forest plots are commonly used in systematic reviews to represent the data synthesis. It is graphical representation of all of the data from quantitative studies.See the example below to get a greater understanding of what they contain:
Acknowledgement: The above course presentation has been adapted from Systematic Reviews library guide by University of South Australia Library.
If the data is heterogeneous a qualitative approach would be required. A common qualitative approach in health sciences is a thematic analysis. "Thematic synthesis has three stages: the coding of text 'line-by-line'; the development of 'descriptive themes'; and the generation of 'analytical themes'." (Thomas & Harden, 2008)
Please consult the following article for an example of how to conduct a thematic analysis.
References:
Thomas, J., & Harden, A. (2008, 2008/07/10). Methods for the thematic synthesis of qualitative research in systematic reviews. BMC Medical Research Methodology, 8(1), 45. https://doi.org/10.1186/1471-2288-8-45