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Systematic reviews provide a method for collating and synthesizing research, and are used to inform healthcare decision making by clinicians, consumers and policy makers. A core component of many systematic reviews is a meta-analysis, which is a statistical synthesis of results across studies. In this review article, we introduce meta-analysis, focusing on the different meta-analysis models, their interpretation, how a model should be selected and discuss potential threats to the validity of meta-analyses. We illustrate the application of meta-analysis using data from a review examining the effects of early use of inhaled corticosteroids in the emergency department treatment of acute asthma.