Data synthesis includes synthesising the findings of primary studies and when possible or appropriate some forms of statistical analysis of numerical data. Synthesis methods vary depending on the nature of the evidence (e.g., quantitative, qualitative, or mixed), the aim of the review and the study types and designs. Reviewers have to decide and preselect a method of analysis based on the review question at the protocol development stage.
In the data synthesis section, you need to present the main findings of your Knowledge synthesis. There are a number of ways in which you can synthesize results from your included studies.
If the studies you have included in your evidence synthesis are sufficiently similar, or in other words homogenous, you can synthesize 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.
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 synthesize 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 standardized, the process underpinning narrative or descriptive synthesis is subjective and there is no one standard process for undertaking this.
In recent years, knowledge syntheses of qualitative research is 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.
Tip: The proposed methods of analysis, type and range of data, and the framework for grouping studies should be planned and documented at the protocol development stage and before conduct. It is advised to consult and ask for help from statisticians for appropriate guidance
(A portion of the above content is courtesy of University of South Australia Library)
Meta-analysis: is a quantitative synthesis of the results from included studies using statistical analysis methods that are extensions to those used in primary studies. Meta-analysis can provide a more precise estimate of the outcomes by measuring and counting for uncertainty of outcomes from individual studies by means of statistical methods. However, it is not always feasible to conduct statistical analyses due to several reasons including inadequate data, heterogeneous data, poor quality of included studies and the level of complexity.
Qualitative Data Synthesis (QDS): is a method of identifying common themes across qualitative studies to create a great degree of conceptual development compared with narrative reviews. The key concepts are identified through a process that begins with interpretations of the primary findings reported to researchers which will then be interpreted to their views of the meaning in a second-order and finally interpreted by reviewers into explanations and generating hypotheses.
Narrative summary: is a summary of the review results when meta-analysis is not possible. Narrative summaries describe the results of the review, but some can take a more interpretive approach in summarizing the results. These are known as "evidence statements" and can include the results of quality appraisal and weighting processes and provide the ratings of the studies.
Mixed methods synthesis: is an advanced method of data synthesis developed by EPPI-Centre to better understand the meanings of quantitative studies by conducting a parallel review of user evaluations to traditional systematic reviews and combining the findings of the syntheses to identify and provide clear directions in practice.
For more information please visit the following Cochrane training links:
Deeks JJ, Higgins JPT, and A. DG, Analysing data and undertaking meta-analyses, in Cochrane Handbook for Systematic Reviews of Interventions version 6.3 T.J. Higgins JPT, Chandler J, Cumpston M, Li T, Page MJ, Welch VA Editor. 2022, Cochrane.
McKenzie JE and B. SE, Synthesizing and presenting findings using other methods, in Cochrane Handbook for Systematic Reviews of Interventions version 6.3 T.J. Higgins JPT, Chandler J, Cumpston M, Li T, Page MJ, Welch VA Editor. 2022, Cochrane
Coren, E. and M.F. Fisher, The conduct of systematic research reviews for SCIE knowledge reviews. 2006, UK: Social Care Institute for Excellence. 85.
Schünemann HJ, et al., Interpreting results and drawing conclusions, in Cochrane Handbook for Systematic Reviews of Interventions version 6.3 T.J. Higgins JPT, Chandler J, Cumpston M, Li T, Page MJ, Welch VA Editor., Cochrane.