» Synthesis of Results
When conducting a systematic review, the methods used for the synthesis of the results will depend on the type of study designs that are included in the review. Synthesis involves the collation, combination and summary of the findings of individual studies.
Most systematic reviews evaluate the effectiveness of interventions from trials of randomised controlled designs and synthesis can be done using quantitative statistical techniques such as meta-analysis. Therefore, a meta-analysis is only appropriate when the studies included are homogenous. However, conducting a review on complex interventions will include a range of study designs that are not quantitatively comparative and result in methodological diversity. Additionally, studies may differ even when they are of the same study design due to variabilities in participants, interventions, and outcomes resulting in clinical diversity. These clinical and methodological diversities in turn result in statistical heterogeneity that make it infeasible for a meta-analysis to be conducted and other approaches for evidence synthesis may need to be considered.
The Cochrane Handbook for Systematic Reviews of Interventions includes chapters on special topics that have been developed to guide the synthesis of different study designs such as cluster randomised trials, non-randomised studies, studies generating qualitative data, and how to approach the issue of heterogeneity.
The EPPI-Centre have also developed guidance in this area, including the following textbook:
An Introduction to Systematic Reviews
Gough D, Oliver S, Thomas J, editors. An introduction to systematic reviews. Sage; 2012 Mar 22
Researchers at LSHTM also contribute to methodological guidance and have produced two text books on systematic reviews:
Synthesising Qualitative and Quantitative Health Evidence: A Guide to Methods: A Guide to Methods
Pope C, Mays N, Popay J. Synthesising Qualitative and Quantitative Health Evidence: A Guide to Methods: A Guide to Methods. McGraw-Hill Education (UK); 2007 Jul 1.
Systematic Reviews in the Social Sciences: A Practical Guide (2006)
Petticrew M, Roberts H. Systematic reviews in the social sciences: A practical guide. John Wiley & Sons; 2008 Apr 15
Stephen Evans at LSHTM was a contributor to a new book on meta-analysis in drug safety published by CIOMS (- The Council for International Organizations of Medical Sciences is an international, non-governmental, non-profit organization established jointly by WHO and UNESCO in 1949). Most meta-analyses concentrate on efficacy of drugs and there are special problems with harms. This book addresses some of the issues.
Evidence Synthesis and Meta-Analysis for Drug Safety
Researchers at LSHTM are also involved in methodological projects, such as:
The MACH (Meta-analysis, Complexity and Heterogeneity) project
When the studies included in a review are not quantitatively comparative, narrative synthesis provides an alternative to meta-analysis in these circumstance. The following is a publication offering guidance on narrative synthesis:
Guidance on the conduct of narrative synthesis in systematic reviews
Popay J, Roberts H, Sowden A, Petticrew M, Arai L, Rodgers M, et al. Guidance on the conduct of narrative synthesis in systematic reviews. ESRC Research Methods Programme; 2006.
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