Once your group has selected the most relevant studies the next step involves extracting the relevant data, and synthesising or compiling your findings using textual or statistical methods.

Your collected data must be combined into a coherent whole and accompanied by an analysis that conveys a clear understanding of the body of evidence via the source material accessed during the review. All reviews should include a qualitative synthesis and may or may not include a quantitative synthesis (also known as a meta-analysis).

Ideally the method of synthesis will have been selected at the design stage of the systematic review. However, it’s a good idea to talk to your group supervisor or project lead. They should be able to provide direction on the most appropriate and effective way for your group to synthesise the results. 

Qualitative Synthesis

A qualitative synthesis is a narrative, textual approach to summarizing, analysing and assessing the body of evidence included in your review. It is a necessary part of all systematic reviews, even those with a focus on quantitative data.

A qualitative synthesis provides the following:

-  A general summary of the characteristics and findings of the included studies.

-  An analysis of the relationships between studies, exploring patterns and investigating heterogeneity.

- Discusses the applicability of the body of evidence to the review's question within the PICO structure.

- Explains the meta-analysis (if appropriate to the study) and interprets and analyses the strength of results. For more information on the meta-analysis process, please view the paragrap below.

- Critiques the strengths and weaknesses of the body of evidence, including a cumulative assessment of the risk of bias across various studies.

- Discusses any gaps in the evidence, such as patient populations that have been inadequately studied or for whom results differ.

- Compares the review's findings with current conventional wisdom when appropriate.

A quantitative synthesis, or meta-analysis, uses statistical techniques to combine and analyse the results of multiple studies. The feasibility and sensibility of including a meta-analysis as part of your systematic review will depend on the data available.

"Meta-analysis is the statistical combination of results from two or more separate studies. Potential advantages of meta-analyses include an improvement in precision, the ability to answer questions not posed by individual studies, and the opportunity to settle controversies arising from conflicting claims" Cochrane Training 2023

The requirements for a quantitative synthesis are as follows:

1. Clinical and methodological similarity between compared studies

2. Consistent study quality among compared studies

3. Statistical expertise from review team members or an external expert.

Visit the following links to view examples of Systematic Reviews which include a Qualitative (Narrative) Synthesis

- Patients′ and clinicians′ experiences of remote consultation? A narrative synthesis by Helen Walthall, Sue Schutz, Joanne Snowball, Raluca Vagner, Nicola Fernandez, Emilia Bartram, Journal of advanced nursing, 07/2022, Volume 78, Issue 7

- Voluntary Work-related Technology Use during Non-work Time: A Narrative Synthesis of Empirical Research and Research Agenda by Svenja Schlachter, Almuth McDowall, Mark Cropley, Ilke Inceoglu, International Journal of Management Reviews, 03 November 2017.

Visit the following links to view examples of Systematic Reviews which include a Quantitative Synthesis

- Seizures in autoimmune encephalitis—A systematic review and quantitative synthesis by Anusha K. Yeshokumar, Arielle Coughlin, Jarrett Fastman, Kendall Psaila, Michael Harmon, Taylor Randell, Emily M. Schorr, Helen Han, Hai Hoang, Celine Soudant, Nathalie Jette. Official Journal of the International League against Epilepsy. 21 January 2021

- Suture techniques during laparoscopic and robot-assisted partial nephrectomy: a systematic review and quantitative synthesis of peri-operative outcomes by Riccardo Bertolo, Riccardo Campi, Tobias Klatte, Maximilian C. Kriegmair, Maria Carmen Mir, Idir Ouzaid, Maciej Salagierski, Sam Bhayani, Inderbir Gill, Jihad Kaouk et al. BJU International, 14 September 2018

- Narrative synthesis

This document provides guidance from the Centre for Reviews and Dissemination for creating a textual summary of the relevant studies and analysing robustness.

- Quantitative synthesis of comparative studies (meta-analysis)

This document also provided by the Centre for Reviews and Dissemination provides information on statistical analysis of the combined results of relevant studies.

- Analysing data and undertaking meta-analyses - Jonathan J. Deeks, Julian P. Higgins, Douglas G. Altman; on behalf of the Cochrane Statistical Methods Group / Cochrane Training Handbook 2023