Literature reviews traditionally introduce a topic, summarise the main issues and provide some illustrative examples. If they are to be considered a reliable source of research evidence they should record how the primary studies were sought and selected and how they were analysed to produce their conclusions. Readers need to be able to judge whether all of the relevant literature is likely to have been found, and how the quality of studies was assessed.

See six reasons below to find out:

'What's so special about a 'systematic' review?'

1) Systematic reviews are transparent about how their conclusions are generated

Why? Because it avoids misrepresentation of the knowledge base

How? Each piece of research is evaluated and its quality and relevance made clear

If we are to be confident about the findings of reviews of research we need to be able to see that review authors have taken steps to reduce distortions or inaccuracies in their work. For instance, were all studies found treated as equally reliable despite differences in their quality; or could some have been missed altogether? A methodical and explicit approach to avoiding ways in which reviews can misrepresent the knowledge base is the fundamental principle of systematic research synthesis.

2) A 'protocol' sets out how the systematic review is to be conducted before the work starts

Why? It reduces bias

How? Because procedures cannot be overly influenced by results

As is the case for any good research, the methods for a systematic review are made explicit in a 'protocol' before it starts. This helps to reduce bias in the review process, for example by ensuring that reviewers' procedures are not overly influenced by the results of studies they find. If changes are needed to the protocol as the review progresses these needed to be noted in the review's final report and the rationale for making changes made clear.
 
In systematic reviews where the nature of the review question requires a review methodology which is more iterative, the method of review will not be fully specified a priori but will be detailed in the final report.

3) Exhaustive searches are undertaken to find as much as possible of the relevant research

Why? It reduces bias

How? Conclusions are not overly influenced by the most accessible research

Systematic reviews include efforts to find as much as possible of the research which addresses the review's research question. This is important if the review's conclusions are not to be over-influenced by studies which are simply the easiest to find (usually published research, showing the benefit of interventions). Another example of the methodological approach of a systematic review is the use of a set of explicit statements, called inclusion criteria, to assess each study found to see if it actually does address a review's research question.

There are some systematic reviews that do not aim to be exhaustive because the nature of their review question and review methods is such that they are only attempting to identify selected examples of evidence. They are systematic reviews as long as there is a transparency, rigour and coherence in the approach used.

4) The systematic review methods are made explicit

Why? So users of the review know if they can trust the review's findings

How? Because readers can judge how well it has been done

A systematic review is also explicit in reporting its methods so that these can be appraised. For example, the methods used to find studies (database searches, searches of specialist bibliographies, hand-searching of likely journals, attempts to track down unpublished research) will be reported in some detail. This allows readers to decide for themselves whether the reviewers have looked carefully enough to be able to say they have identified as many as possible of the studies that could help answer the review's research question. It is now standard practice for reports of systematic reviews to have clearly defined methods and results sections.

5) Potential users of the systematic review are involved

Why? To make sure that the research is relevant

How? Advisory groups are set up with representatives of all user groups

In order to meet the needs of all potential users of research, syntheses need to involve a broad range of users in the development of review questions and procedures. Advisory groups can assist with defining the broad topic area to be looked at and identifying the specific areas within that topic that would be most useful to scrutinise in-depth.

6) The findings of sound research are synthesised

Why? To produce clear and easily accessible messages about the reliable evidence available on a given topic

How? Appraising individual studies and pooling their results means conclusions can be drawn about the direction of the evidence as a whole

An important characteristic of a systematic review is that it includes a synthesis of its results, which in this case are results from previous research.

As a very important part of the synthesis process, systematic reviewers assess the quality of the studies they have found. They can then use this assessment to assign different weights to study findings. Poor quality studies are sometimes downgraded in importance or excluded from the review. The ultimate effect of this is that research can influence a review's conclusions only when that research is sound.

The synthesis is usually presented in the form of a structured narrative, summary tables or a statistical combination (meta-analysis). This synthesis is then used to formulate conclusions and recommendations. The aim is to make clear the links between the detail of the studies found and the reviewers' conclusions.

EPPI Centre methods references