A systematic review is a very specific type of study which attempts to bring together all the evidence which meets a predetermined specified criteria to answer a particular research question. Such studies are considered a high form of evidence and sit at or near the top of the hierarchy of evidence.
One of the most respected systematic review organisations is Cochrane which defines systematic reviews in the following way:
A systematic review attempts to identify, appraise and synthesize all the empirical evidence that meets pre-specified eligibility criteria to answer a specific research question. Researchers conducting systematic reviews use explicit, systematic methods that are selected with a view aimed at minimizing bias, to produce more reliable findings to inform decision making.
Many systematic reviews looking at the effectiveness of an intervention or the effectiveness of a diagnostic tool only consider higher level evidence such randomised controlled trials (RCTs) and carry out a meta-synthesis of a number of studies in order to come to a more powerful conclusion. However, meta-synthesis is not always possible or appropriate because of significant heterogeneity between studies.
Other systematic reviews exist do not attempt to carry out a meta-synthesis because they are addressing different types of questions such as the views and feelings of healthcare professionals or patients about a particular intervention. They will therefore be considering a wider range of studies including ones which have used a qualitative methodology.
There are a number of organisations which both co-ordinate systematic reviews and provide training and support for those undertaking them. The most well known is the Cochrane Collaboration. Others include Campbell, EPPI Centre and the Joanna Briggs Institute.
These organisations are a very useful source of information on good practice in terms of the conduct of a systematic review. One extremely useful source is the Cochrane Handbook. There is also the Methodological Expectations of Cochrane Intervention Reviews (MECIR). These are a set of standards to which all Cochrane reviews need to adhere to.
Another very useful source for good practice in terms of systematic review conduct and what should be included in a systematic review protocol and the completed systematic review is PRISMA (Prefered Reporting Items for Systematic Reviews and Meta-Analysis). PRISMA is primarily for systematic reviews that are evaluating RCTs or reviews on interventions which consider other types of studies .