Synthesize the results
The final step is to synthesize, map, or describe the data that you have collected in your final manuscript. There are a number of ways in which you can synthesize the results of your included studies.
If the studies that you have included are sufficiently similar, or in other words have a sufficiently low level of heterogeneity, one of the ways you can synthesize the data from these studies is via a process called meta-analysis. A meta-analysis is a statistical approach to bring together results from multiple studies.
Resources for meta-analysis and other qualitative and quantitative evidence:
Borenstein, M., Hedges, L., Higgins, J., & Rothstein, H. (2021). Introduction to meta-analysis (Second ed.). Hoboken, NJ: John Wiley & Sons. (2009 edition is available online)
- Search for additional books and chapters on conducting meta-analyses
- If there is a high level of heterogeneity in your results, or in other words they are not similar (for example, due to different research designs of t thihe included studies), a meta-analysis is not possible. In this situation, you would undertake a narrative or descriptive synthesis. The process for this type of synthesis is subjective and there is no one standardized process.
Resources for narrative synthesis:
Thomson H, Campbell M. “Narrative synthesis” of quantitative effect data in Cochrane reviews: Current issues and ways forward. Cochrane Learning Live Webinar Series 2020 Feb.
- Part 1 helps navigate some of the confusion over the concepts of "narrative synthesis" or "qualitative review of (quantitative) data" versus the ambiguous use of the terms "narrative review" or "qualitative review"
Campbell, M., McKenzie, J. E., Sowden, A., Katikireddi, S. V., Brennan, S. E., Ellis, S., Hartmann-Boyce, J., Ryan, R., Shepperd, S., Thomas, J., Welch, V., & Thomson, H. (2020). Synthesis without meta-analysis (SWiM) in systematic reviews: Reporting guideline. BMJ, 368, l6890. doi:10.1136/bmj.l6890
Resources for Network meta-analysis:
Hoaglin DC, Hawkins N, Jansen JP, Scott DA, Itzler R, Cappelleri JC, et al. Conducting indirect-treatment-comparison and network-meta-analysis studies: Report of the ISPOR Task Force on Indirect Treatment Comparisons Good Research Practices: Part 2. Value Health. 2011;14(4):429-37.