Systematic reviews are the best way to bring together and synthesize scientific evidence on a subject by applying a method that ensures minimization of bias and study shortcomings. This was mentioned in a previous article of this series. This goal can be achieved by recurring to the principles of the scientific method. The research subjects are not patients but published clinical trials and other sources from which data are extracted. In the beginning, these studies were limited to therapeutic effectiveness and other forms of intervention. Currently, systematic reviews on prognostic and etiological issues are commonly done. Systematic reviews are comprised by the following steps: formulation of the review question; definition of sources and search methods; definition and application of the trial selection criteria; critical analysis of selected studies; pooling and synthesis of trial data; and conclusions by inference. Lastly, a meta-analysis is a systematic review in which primary trial results are combined with statistical methods.