Synthesizing Study Results in a Systematic Review

By Verbeek, Jos; Ruotsalainen, Jani et al. | Scandinavian Journal of Work, Environment & Health, May 2012 | Go to article overview

Synthesizing Study Results in a Systematic Review


Verbeek, Jos, Ruotsalainen, Jani, Hoving, Jan L., Scandinavian Journal of Work, Environment & Health


A single study rarely suffices to underpin treatment or policy decisions. This creates a strong imperative for systematic reviews. Authors of reviews need a method to synthesize the results of several studies, regardless of whether or which statistical method is used. In this article, we provide arguments for combining studies in a review. To combine studies, authors should judge the similarity of studies. This judgement should be based on the working mechanism of the intervention or exposure. It should also be assessed if this mechanism is similar for various populations and follow-up times. The same judgement applies to the control interventions. Similar studies can be combined in either a meta-analysis or narrative synthesis. Other methods such as vote counting, levels of evidence synthesis, or best evidence synthesis are better avoided because they may produce biased results. We support our arguments by re-analysing a systematic review. In its original form, the review showed strong evidence of no effect, but our re-analysis concluded there was evidence of an effect. We provide a flowchart to guide authors through the synthesis and assessment process.

Key terms clinical heterogeneity; intervention; knowledge translation; levels of evidence; meta-analysis; occupational health.

The basic idea underlying evidence-based medicine is that better use of evidence from scientific research will increase the quality of healthcare including prevention (1). Evidence is, however, seldom unequivocal and many topics of interest to practitioners have been evaluated in more than one study with varying results. This creates a clear need for synthesizing the results of multiple studies such as in systematic reviews. The systematic review has been defined as a review in which bias has been reduced by the systematic identification, appraisal, synthesis, and, if relevant statistical aggregation of all relevant studies on a specific topic according to a predetermined and explicit method (2). The value of systematic reviews in providing answers to questions relevant to practice is increasingly recognized also for occupational health (3, 4).

In the past, rather than providing an answer to a specific question, the purpose of a review was to give an overview of what had been written about a certain topic in the scientific literature. For this traditional "overview type" of review, synthesis of the results in one summary outcome is less necessary. This difference in objectives has created confusion about if, when, and how results of studies in reviews should be synthesized.

Not all types of questions can be answered with systematic reviews. The traditional idea of giving an overview of "the state of the art" can still be useful. It is, however, increasingly recognized that also in this respect it would be good to be more systematic. This has led to a new nomenclature for reviews such as "scoping" reviews (5). The objective of a scoping review is to summarize a range of evidence in order to convey the breadth and depth of a field. Such reviews have requirements different than systematic reviews as defined above. Results of qualitative studies can also be combined in a synthesis of studies, but the problems here are different from those in quantitative studies (6). Therefore, in this article, we restrict ourselves to systematic reviews of quantitative studies only.

There is sometimes confusion about the difference between a systematic review and a meta-analysis. A systematic review is a review of the literature, but it does not necessarily include a meta-analysis. A metaanalysis is a statistical synthesis of the results of several individual studies in one pooled summary estimate. As such, it is easy to see that a meta-analysis requires a systematic review of the literature. Since a meta-analysis is often included in a systematic review, many use the term meta-analysis as a synonym for systematic review (7). Meta-analysis has a long history in educational and psychological research (8). …

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