An introduction to systematic reviews in respiratory medicine

Reviews are an essential component of modern medicine. The volume of literature is large, even about a single treatment for pulmonary disease. The task of retrieving all the relevant papers, then assessing the evidence to reach a valid conclusion is very time consuming. At every stage there is the risk of sampling error (failure to get all the evidence) and bias (a systematic distortion of the results due to a weakness in the methodology). There are essentially two types of review: narrative reviews that follow no rules, exposing them to sampling error and bias; and systematic reviews that attempt to minimise these effects by following an explicit structure for retrieving all of the evidence and attempting an objective synthesis of the results from the different trials. A good review can serve a number of purposes including: assembling all the relevant evidence in one place, providing a valid estimate of the overall effect of treatment, producing guidance for clinical practice and generating hypotheses for further trials about patients or settings in which the treatment effect may be less or more effective.