Systematic reviews are a type of literature review of research which require equivalent standards of rigour as primary research. They have a clear, logical rationale that is reported to the reader of the review. They are used in research and policymaking to inform evidence-based decisions and practice. They differ from traditional literature reviews particularly in the following elements of conduct and reporting.
For example, systematic reviews (like all research) should have a clear research question, and the perspective of the authors in their approach to addressing the question is described. There are clearly described methods on how each study in a review was identified, how that study was appraised for quality and relevance and how it is combined with other studies in order to address the review question. A systematic review usually involves more than one person in order to increase the objectivity and trustworthiness of the reviews methods and findings.
Research protocols for systematic reviews may be peer-reviewed and published or registered in a suitable repository to help avoid duplication of reviews and for comparisons to be made with the final review and the planned review.
Literature reviews provide a more complete picture of research knowledge than is possible from individual pieces of research. This can be used to: clarify what is known from research, provide new perspectives, build theory, test theory, identify research gaps or inform research agendas.
A systematic review requires a considerable amount of time and resources, and is one type of literature review.
If the purpose of a review is to make justifiable evidence claims, then it should be systematic, as a systematic review uses rigorous explicit methods. The methods used can depend on the purpose of the review, and the time and resources available.
A 'non-systematic review' might use some of the same methods as systematic reviews, such as systematic approaches to identify studies or quality appraise the literature. There may be times when this approach can be useful. In a student dissertation, for example, there may not be the time to be fully systematic in a review of the literature if this is only one small part of the thesis. In other types of research, there may also be a need to obtain a quick and not necessarily thorough overview of a literature to inform some other work (including a systematic review). Another example, is where policymakers, or other people using research findings, want to make quick decisions and there is no systematic review available to help them. They have a choice of gaining a rapid overview of the research literature or not having any research evidence to help their decision-making.
Just like any other piece of research, the methods used to undertake any literature review should be carefully planned to justify the conclusions made.
Finding out about different types of systematic reviews and the methods used for systematic reviews, and reading both systematic and other types of review will help to understand some of the differences.
Typically, a systematic review addresses a focussed, structured research question in order to inform understanding and decisions on an area. (see the Formulating a research question section for examples).
Sometimes systematic reviews ask a broad research question, and one strategy to achieve this is the use of several focussed sub-questions each addressed by sub-components of the review.
Another strategy is to develop a map to describe the type of research that has been undertaken in relation to a research question. Some maps even describe over 2,000 papers, while others are much smaller. One purpose of a map is to help choose a sub-set of studies to explore more fully in a synthesis. There are also other purposes of maps: see the box on systematic evidence maps for further information.
Reporting standards specify minimum elements that need to go into the reporting of a review. The reporting standards refer mainly to methodological issues but they are not as detailed or specific as critical appraisal for the methodological standards of conduct of a review.
A number of organisations have developed specific guidelines and standards for both the conducting and reporting on systematic reviews in different topic areas.
PRISMA is a reporting standard and is an acronym for Preferred Reporting Items for Systematic Reviews and Meta-Analyses. The Key Documents section of the PRISMA website links to a checklist, flow diagram and explanatory notes.
PRISMA is less useful for certain types of reviews, including those that are iterative.
eMERGe is a reporting standard has been developed for Meta-Ethnographies, further details are available from the project website