How to map the evidence: the development of the systematic review in anaesthesia

The decision of where to start a research project has been influenced by many factors over the years. Tradition has a large impact, but the individual researchers' or clinicians' personal interest has also played a major role. The pharmaceutical industries' interest has without doubt initiated and sponsored many projects in order to get new products onto the market. The lack of an overview and control has led to an abundance of evidence within certain areas of our specialty, whereas other areas are scarcely, or not at all, researched. One way of 'mapping' the evidence in order to find out what we know and what we do not know is the production of systematic reviews. Although systematic reviews are considered top of the evidence hierarchy, they are not flawless. The aim of this article is to explain the systematic review and point to some of the challenges in the development and use of systematic reviews.