Synthesis and analysis
The sort of synthesis and analysis you will conduct will depend on the nature of the question considered in your review, the types of study included and the sorts of data collected. You may carry out one of these:
When studies range in design and outcome, a statistical analysis is not always appropriate or possible; a narrative synthesis provides a summary and explanation of the results in word form.
A meta-analysis is a statistical method for combining and analysing the numerical results of different studies included in the review. It is commonly used in systematic reviews comprising randomised controlled trials. The quality of the meta-analysis is, however, very much dependent on the quality of the search and data extraction which proceed it. If relevant studies have been missed, their quality has not been properly assessed or their data has been misinterpreted this will have a negative impact on any meta-analysis (see Crombie and Davies 2009).
In simple terms, meta-aggregation can be viewed as a qualitative equivalent to meta-analysis - findings from qualitative research studies (rather than numerical data from quantitative studies) are brought together, categorised into groups "on the basis of similarity in meaning. These categories are then subjected to a meta-synthesis in order to produce a single comprehensive set of synthesized findings that can be used as a basis for evidence-based practice" (Lookwood and Pearson 2013 chapter 4; see also Munn, Tufanaru and Aromataris 2014 for a worked example).
For further information on types of data synthesis see:
- Centre for Reviews and Dissemination's guidance (section 1.3.5)
- The Cochrane Handbook (chapter 9)
- Joanna Briggs Institute Reviewers’ Manual
- JBI's Systematic Reviews: Data Extraction and Synthesis (Munn, Tufanaru and Aromataris 2014)
You are now ready to write up your systematic review but may be wondering what sort of format to follow.
If you are an academic conducting a review for a particular body you should of course adhere strictly to any guidance and format they provide; if you are a student you may already have been given instructions in your assignment brief (or also been referred to particular formats used by review bodies).
The Centre for Reviews and Dissemination's guidance for undertaking reviews (2009) (section 1.3.6) contains a suggested structure for a review report as well as useful advice in relation to all parts of the report. Your report will generally contain:
- Title and authors
- Executive summary or abstract
- Review question
- Methods (of search, selection of studies, assessment of quality, data extraction, synthesis)
- Results, discussion, conclusions and (if any) recommendations for further research
You may wish to consider one of the following styles:
- PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) 2009 checklist
- Methodological Expectations of Campbell Collaboration (C2) Intervention Reviews (MEC2IR)
- Cochrane Handbook (see chapters 4, 9, 10, 11 and 12)
- Equator Network (Enhancing the QUAlity and Transparency Of health Research) (provides links to additional possible formats)
When writing your review, it is important to be honest about any possible issues of bias (for example language bias (if you included only evidence in the English language) and any flaws in the studies included in the review (and their impact on the weight you have ascribed to them).