Systematic Reviews and Other Evidence Synthesis Types Guide

Task 13 – Synthesize Data

Option 1

The ES/SR Team outlines the synthesis plan in the protocol. Synthesis is a process of bringing together data from a set of included studies with the aim of drawing conclusions about a body of evidence. Chapter 9 of the Cochrane Handbook of Systematic Review Interventions includes a general framework for synthesis that can be used to guide the process of planning the comparisons, preparing for synthesis, undertaking the synthesis, and interpreting and describing the results (Higgins JPT, 2020).

The synthesis may be descriptive. For example, a qualitative synthesis describes study methodology, strengths and limitations of individual studies, patterns across studies, potential bias, relationships between the characteristics, and relevance of individual studies to the populations, comparisons, cointerventions, settings, and outcomes or measures of interest.

Option 2

The ES/SR Team may decide that the collected data is sufficiently homogeneous and a meta-analysis can be performed (Task 14). By combining the results of multiple studies in a meta-analysis, the increased number of study participants can reduce random error, improve precision, and increase the likelihood of detecting a real effect (IOM (Institute of Medicine), 2011; Muka et al., 2020).

Task 14 – Meta-Analyze

In this task, the ES/SR Team must determine whether to combine the collected data and perform a meta-analysis. Potential advantages of meta-analyses include:

1. Improving precision – Many studies are too small to provide convincing evidence about intervention effects in isolation. By combining data, the sample size is increased and the estimation is usually improved.
2. Answering questions not posed by the individual studies – Primary studies often involve a specific type of participant and explicitly defined interventions. A selection of studies in which these characteristics differ can allow investigation of the consistency of effect across a wider range of populations and interventions. It may also, if relevant, allow reasons for differences in effect estimates to be investigated.
3. Resolving uncertainty when primary studies disagree or generate new hypotheses – The statistical synthesis of findings allows the degree of conflict to be formally assessed, and reasons for different results to be explored and quantified.

However, meta-analysis also has the potential to mislead, particularly if specific study designs, within-study biases, variation across studies, and reporting biases are not carefully considered (Higgins JPT, 2020; IOM (Institute of Medicine), 2011; Muka et al., 2020).

Task 15 – Re-Check Literature

If the initial search date is more than 12 months – but preferably six months – from the intended publication date, the Librarian should re-run the searches in the selected databases and non-database sources. The results should be screened to identify potentially eligible studies and included in the review. If not, then the potentially eligible studies will need to be reported as references under "Studies awaiting classification" or "Ongoing studies" if they are not yet completed. (Higgins JPT, 2020)