Systematic literature reviews (SLRs)
SLR’s attempt to collate all empirical evidence that fit pre-specified eligibility criteria in order to answer a specific clearly-formulated research question. A SLR uses explicit and reproducible systematic methods that are selected with a view to minimizing bias, thus providing more reliable findings from which conclusions can be drawn and decisions made.
The process starts with a research question and a protocol or research plan. A review team searches for studies to answer the question using a highly sensitive search strategy. The retrieved studies are then screened for eligibility using pre-specified inclusion and exclusion criteria (this is done by at least two people working independently). Next, the reviewers extract the relevant data and assess the quality of the included studies. Finally, the review team synthesizes the extracted study data and presents the results.
A SLR may contain meta-analyses (statistical analysis). A SLR which is continually updated, incorporating relevant new evidence as it becomes available is often known as a living SLR.
Rapid reviews aim to produce a rigorous synthesis quickly (due to time constraints/urgency), based on a pre-defined research question. The review process for rapid reviews is the same as for a more traditional systematic review: the emphasis is on a replicable pre-specified search, and screening methods that minimize the risk of bias, although potentially isn’t as stringent as a formal systematic review.
The process operates within pre-specified limits (for example, by restricting searches to articles published during a specific timeframe) and is usually run by a multidisciplinary team with expertise in systematic review methods.
Umbrella reviews or Overview of reviews
An umbrella review is a review of multiple systematic reviews. The process uses explicit and systematic methods to search for, and identify, systematic reviews on related research questions in the same topic area. The purpose of an umbrella review is to synthesize the results of the systematic reviews across important outcomes.
Scoping reviews are exploratory and they typically address a broad question, compared to a systematic review that typically has a more targeted question.
Researchers conduct scoping reviews to assess the extent of the available evidence, to organize it into groups and to highlight gaps. If a scoping review finds no studies, this might help researchers to decide that a systematic review is likely to be of limited value and that resources could be better directed elsewhere.
Literature reviews or narrative reviews
Literature, or narrative, reviews provide an overview of what is known about a particular topic. They evaluate the material, rather than simply restating it, but the methods used to do this are not usually prespecified and they are not described in detail in the review. The search might be comprehensive but it does not aim to be exhaustive. Literature reviews are often topic based and can take the form of a discussion. Literature reviews lack precision and replicability and can present their findings in the context of what has come before. Often, this sort of synthesis does not attempt to control for the author’s own bias. The results or conclusion of a literature review is likely to be presented in a narrative format rather than statistical methods.
Take a look at the articles about the different types of review on the Covidence blog: