Synthesizing study results in a systematic review.
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 is evidence of an effect. We provide a flow-chart to guide authors through the         synthesis and assessment process.
Finnish Institute of Occupational Health, Cochrane Occupational Safety and Health Review Group, Kuopio, Finland. jos.verbeek@ttl.fi
Jos
J
Verbeek
Jani
J
Ruotsalainen
Jan L
JL
Hoving
D015982:Bias; D003627:Data Interpretation, Statistical; D003657:Decision Making; D019317:Evidence-Based Medicine; D006801:Humans; D015201:Meta-Analysis as Topic; D016272:Occupational Health / Q000706:statistics & numerical data*; D012107:Research Design / Q000706:statistics & numerical data*; D012196:Review Literature as Topic