Data synthesis
You've reached one of the final stages of the review with the work done in data extraction and critical appraisal coming together to form the data synthesis.
The method for your synthesis will depend on the aim, scope and data found in your review. You will need to select specific approaches depending on the kind of data you are working with. All meta-analsyses are usually systematic reviews, but not all systematic reviews require or suit a meta-analysis. Too much heterogeneity in quantitative data will mean you are unable to do a meta-analysis. A narrative synthesis may be a better approach for many reviews as not all reviews are focused on a quantitative summary of treatment effects/study results. Meta-analyses will also require some level of narrative synthesis as well. Reviews examining qualitiative studies will require a qualitative synthesis method.
An additional step for some reviews will be an assessment of evidence certainty, which is dealt with in the subsection of this chapter.
Synthesis methods
- Chapters 9-11: Cochrane Handbook for Systematic Reviews of Interventions version 6.4These chapters in the Handbook provide guidance on preparing for synthesis, doing meta-analyses and network meta-analyses.
- Chapter 12: Cochrane Handbook for Systematic Reviews of Interventions version 6.4This section of the handbook examines why a meta-analysis of effect estimates may not be possible and focuses on the other statistical synthesis methods, along with approaches to tabulating, visually displaying and providing a structured presentation of the findings.
- 3.5.1 Narrative synthesis: Systematic Reviews: CRD's guidance for undertaking reviews in health careThis chapter explains how to do a narrative synthesis for quantitative data - creating a textual summary of the relevant studies and analysing their robustness.
- Forero DA, Lopez-Leon S, González-Giraldo Y, Bagos PG (2019) Ten simple rules for carrying out and writing meta-analyses. PLoS Comput Biol 15(5)This short but helpful article presents ten simple rules for carrying out and writing meta-analyses and is a good place to start before exploring the more detailed guidance available.
- SWiM (Synthesis without meta-analysis)While SWiM is a reporting guideline, rather than a methodological guideline, you will need to understand what aspects of a narrative synthesis you will need to report on. The Webinars and Resources section of the website has some useful material for further understanding of this type of synthesis.
Meta-Analysis with R by ISBN: 3319214160Publication Date: 2015This book provides a comprehensive introduction to performing meta-analysis using the statistical software R. As such, the book introduces the key concepts and models used in meta-analysis. It also includes chapters on the following advanced topics: publication bias and small study effects; missing data; multivariate meta-analysis, network meta-analysis; and meta-analysis of diagnostic studies.
Available from Imperial Library - print and eBook
Doing Meta-Analysis with R by ISBN: 9780367610074Publication Date: 2021Contains two introductory chapters on how to set up an R environment and do basic imports/manipulation of meta-analysis data, including exercises. Describes statistical concepts clearly and concisely before applying them in R. Includes step-by-step guidance through the coding required to perform meta-analyses, and a companion R package for the book.
Available from Imperial Library in Print. The online version is available at https://bookdown.org/MathiasHarrer/Doing_Meta_Analysis_in_R/
While some sources will refer to a predominently textual summary of quantitative evidence as a "qualitative synthesis" or "qualitative evidence summary" this is no longer the accepted terminology and this guide only uses the term qualitiative in the context of data synthesis to refer to that which deals with qualitiative data.
- Section 8. Mixed methods systematic reviews: JBI manual for evidence synthesisInformation on the methodological approaches for conducting a synthesis with quantitative and qualitative evidence.
- 6.5 Synthesis of Qualitative Research: Systematic Reviews: CRD's guidance for undertaking reviews in health careThis section of the CRD Guidance introduces various methods for the synthesis of qualitative evidence.
- Introduction to qualitative research and qualitative evidence synthesis Part 1From the Cochrane Qualitative and Implementation Methods Group", part 1 of this webinar is "What is qualitative evidence synthesis and why we do it".
- Introduction to qualitative research and qualitative evidence synthesis Part 2From the Cochrane Qualitative and Implementation Methods Group", part 2 of this webinar is "Methodologies, methods and processes".
