Systematic reviews and meta-analysis
A systematic review attempts to identify, appraise and synthesise all the empirical evidence that meets pre-specified eligibility criteria to answer a given research question. Researchers conducting systematic reviews use explicit methods aimed at minimising bias, in order to produce more reliable findings that can be used to inform decision making (Cochrane Handbook for Systematic Reviews of Interventions).
A systematic review asks a clear, focused question and describes how the authors identified, appraised and synthesised the studies relevant to that question, minimising biases in the collation processes. Others should be able to reproduce the review. Systematic reviews are seen as the mainstay of evidence-based practice because there are often several, and sometimes many, effectiveness studies relevant to any given clinical or public health practice question. They do not have to be exhaustive, but should provide a search strategy and clear inclusion and exclusion criteria to account for the studies they have considered. For example, a review may only include randomised controlled trials (RCTs) or only studies published in the last 10 years. A systematic review can include any type of study; the approach is not confined to RCTs, but the quality and relevance of the studies should be appraised. Finally, a systematic review provides a synthesis of the included studies, which may be a quantitative pooling of the results by meta-analysis or a narrative synthesis or a combination of both.
The results of a systematic review are usually summarised in a Forest plot, where the individual study results are plotted on a vertical axis (Figure 12a, items 1–3). Such a plot may not be possible where different studies use diverse outcome measures.