The terms “systematic review” and “meta-analysis” often go hand-in-hand. However, there are other ways to synthesize and present the findings of a systematic review that do not entail statistical pooling of the data. This is referred to as narrative synthesis or Synthesis Without Meta-Analysis (SWiM), and a recent webinar presented by Cochrane (viewable for free here) provided the definition, potential uses, and pitfalls to watch for when considering the use of narrative synthesis within a systematic review.
What is narrative synthesis/SWiM?
Narrative synthesis or Synthesis Without Meta-analysis (SWiM) is an approach used to describe quantitatively reported data from studies identified within a systematic review in a way that does not quantitatively pool or meta-analyze the data. Narrative synthesis is not the same as a narrative review, which is an unsystematic approach to gathering studies.
A narrative synthesis adds value to the literature by providing information about what the studies on a certain topic say as a whole, as opposed to simply summarizing the findings from individual studies one-by-one. Whereas a meta-analysis is useful in that it provides an overall estimate of the size of an effect of an intervention, a narrative synthesis allows the reviewer to organize, explore, and consider the ways that the findings from several studies are connected to as well as how they are different from one another – and the potential moderators that define these relationships. Thus, its focus is on the question of the existence, nature, and the direction of an effect, rather than its size.
When is it appropriate to perform a narrative synthesis/SWiM?
There are several reasons why narrative synthesis/SWiM may be used when reporting the findings of a systematic review.
· There are not enough data to calculate standardized effect sizes. Meta-analyzing outcomes that are reported using different scales requires the standardization of these data. However, in certain fields, authors of studies may be less likely to report all of the elements required to calculate a standardized effect size, such as the measures of variance; contacting the authors to obtain this information may not yield the needed data. To exclude these studies outright, however, and meta-analyze only studies in which all the needed data are reported, may under- or misrepresent the entire body of evidence.
· There is substantial heterogeneity among included studies. Notable inconsistency between studies with regards to their effect sizes and direction (statistical heterogeneity), the study design (methodological heterogeneity), or from clinical differences surrounding the PICO may render a quantitative meta-analysis of studies to be of little utility, especially if there is a small number of studies to be analyzed together. However, it’s important to ask yourself whether the heterogeneity is truly of enough concern to preclude meta-analysis. PICO elements should be carefully considered a priori as to which are similar enough to be pooled, and which require their own analysis.
What are some common errors made in narrative syntheses/SWiMs?
There are a few common piftalls to watch out for when deciding to report your synthesis without quantitative meta-analysis.
· Not transparently reporting that a narrative synthesis was used when data could not be/were not meta-analyzed
· Not reporting the methods used for narrative synthesis in detail
· Not referring to methodological guidance when describing the decision to perform a narrative synthesis
· Not providing clear links between the data and the synthesis, such as via tables or charts used to report the same data as in the text.
By improving the reporting and presentation of these items within a systematic review, end-users will be better able to understand the reasons why a narrative synthesis was conducted, and ultimately utilize the findings.
Guidance for the reporting of narrative synthesis, or SWiMs, can be found by using the new SWiM reporting guideline checklist here.
We recently reported on GRADE guidance for assessing the certainty of evidence in such circumstances as when a narrative synthesis is presented. More here.