Synthesising
Learning Objectives
This chapter will help you:
- consider how to combine and analyse extracted data.
- identify ways to display the results.
“Synthesis is a process of bringing together data from a set of included studies with the aim of drawing conclusions about a body of evidence.” (McKenzie et al., 2023, s. 9.1)
Synthesis and analysis consider the included studies as a whole (Booth et al., 2022). How you synthesise the results of your included studies will depend on your review type, question, and aim, as well as the methodology of your included studies. You will need to examine the characteristics of each study, for example, the study design, demographics of the participants, or the intervention being tested.
This short video by Cochrane gives an overview of synthesising data for a systematic review.
Cochrane Crowd. (2023, September 28). Module 4: Synthesising the evidence [Video]. YouTube. https://youtu.be/Ffpc9bX4sNE
Synthesis methods
If all included studies are quantitative, you can consider how similar and different the elements are such as settings or interventions. This helps you decide whether you synthesise all the studies together, or group them so the most similar ones are synthesised together (Booth et al., 2022). Depending on your review type, you may need to conduct statistical analyses as well.
For qualitative studies, the patterns you seek to identify are the concepts, themes, and ideas in your included studies. You can compare and contrast the studies and potentially find reasons why there may have been different findings.
If your review includes both qualitative and quantitative studies, it can be challenging to synthesise data from very different study types. However, it can be useful for complex questions. You may synthesise the qualitative and quantitative studies separately before combining them. You could also conduct different reviews of each type of study.
Displaying the synthesis
Some of the ways you can display the synthesis are:
- text: This is descriptive, and summarises and explains studies in words. It is useful when studies are too different for statistical analyses and can include comparing and contrasting elements of studies, such as participants or settings.
- tables: This is also descriptive and can compare elements of the studies. It can often be easier for the reader to see the patterns. You may include themes or details of the studies.
- numerical: This uses statistics and can combine the results of individual studies (a meta-analysis). The studies need to be as similar as possible (homogeneous) for this to be accurate and reliable. The statistics can also be descriptive, such as the number of studies from different countries.
- graphs: This can help identify patterns and present the data visually. There are many types of graphs and you need to choose the most appropriate one (Booth et al., 2022).
Analysis
After the synthesis, you will consider what it means regarding your review question. Think about how your findings can be applied practically and how they relate to current literature and practice. You could discuss the critical appraisal, any data commonly missing from the included studies, and the limitations of the review or the included studies. Consider what these factors could mean for the results of your review. You may discuss gaps in the evidence you have found, or the reasons for differing findings of studies (Booth et al., 2022; Tufanaru et al., 2020).
Example
See below for the data synthesis methods of the following systematic review.
Sullivan, O., Curtin, M., Flynn, R., Cronin, C., Mahony, J. O., & Trujillo, J. (2024). Telehealth interventions for transition to self-management in adolescents with allergic conditions: A systematic review. Allergy, 79, 861-883. https://doi.org/10.1111/all.15963
Further reading
DistillerSR. (n.d.). Systematic review discussion example. About systematic reviews. https://www.distillersr.com/resources/systematic-literature-reviews/systematic-review-discussion-example
References
Booth, A., Sutton, A., Clowes, M., & Martyn-St James, M. (2022). Systematic approaches to a successful literature review (3rd ed.). SAGE.
McKenzie, J. E., Brennan, S. E., Ryan, R. E., Thomson, H. J., & Johnston, R. V. (2023, August). Summarizing study characteristics and preparing for synthesis. In J. P. T. Higgins, J. Thomas, J. Chandler, M. Cumpston, T. Li, M. J. Page, & V. A. Welch (Eds.), Cochrane handbook for systematic reviews of interventions (version 6.4.). Cochrane. https://training.cochrane.org/handbook/current/chapter-09
Tufanaru, C., Munn, Z., Aromataris, E., Campbell, J., & Hopp, L. (2020). Systematic reviews of effectiveness. In E. Aromataris, & Z. Munn (Eds.), JBI manual for evidence synthesis. https://jbi-global-wiki.refined.site/space/MANUAL/355827955/4.+Systematic+reviews+of+effectiveness
How research is done, including how information is collected and analysed and why a particular method was chosen.
Something that aims to make a change and is tested through research.
Quantitative research uses statistical methods to count and measure outcomes from a study. The outcomes are usually objective and predetermined.
Qualitative research uses individual in-depth interviews, focus groups or questionnaires to collect, analyse and interpret data on what people do and say.
The process of assessing and interpreting evidence, by systematically considering its validity, results and relevance to your own context.
The practical or theoretical shortcomings of a study that are often outside of the researcher's control.
