In general, a systematic review describes the rigorous process of systematically collecting, synthesizing, and reporting data to answer a well-defined research question. One of the first times we saw something resembling a “systematic review” in history was in 1753 when James Lind reported on all the known unbiased evidence on scurvy. However, systematic reviews truly rose to prominence in the 1970s and 1980s with the help of Archie Cochrane and his influential texts. The demand for quality evidence-based research has continued to grow, and today we see “systematic reviews” in some form or another across numerous industries and research fields.
Systematic reviews impact a vast number of industries and job roles, whether you’re a medical device professional, pharmacovigilance specialist, academic researcher, regulatory affairs expert, epidemiologist, risk analyst, or healthcare worker. In these fields and many more, your job is probably somehow influenced by the results of systematic reviews, even if you're not the individual conducting the reviews.
Although systematic reviews are seen throughout the global research community, there are a diverse range of terms and definitions used to describe them. Terms can change over the years and vary across research fields. In this post, we’re taking a general look at some of the most common types of reviews, focusing on those that are defined (at least in some capacity) by a systematic approach:
1. Systematic Reviews
A systematic review is the process of systematically searching, gathering, synthesizing, and reporting data to answer a specific, well-defined research question. Within this type of review, we also see sub-categories including interventional reviews (examining whether or not a particular intervention works for a specific outcome), diagnostic reviews (examining how diagnostic tests work for patients), prognostic reviews (attempting to efficiently and accurately predict disease outcomes) and many others.
Best practices may differ between organizations and fields (e.g. the Cochrane Collaboration has its own preferred best practices compared to other organizations) but typically recommend dual independent screening practices with clearly defined criteria to inform the exclusion or inclusion of references, assessment of the quality or risk of bias of included studies, and a synthesis of evidence.
These reviews are typically done to assess what is currently known and develop recommended practices based on that information. They are also used to identify uncertainty in topics and recommend future research. Results are usually presented narratively, in tables, and graphically using charts such as forest plots.
2. Literature Reviews
The name “literature review” is also sometimes used interchangeably with “systematic review” or “systematic literature review” depending on the industry. For example, “literature review” or “systematic literature review” are used widely in regulatory and medical device research to describe what is often a systematic review of the evidence although there may be differences in approaches. This is somewhat confusing because in other fields, a literature review may be used to describe expert reviews that are conducted non-systematically. Quality assessment is not always definite in a literature review and results are presented narratively in most cases.
3. Umbrella Reviews
Umbrella reviews summarize data from multiple systematic reviews, rather than looking at primary studies. You can consider them to be a “review of reviews” or “overview of reviews.” Umbrella reviews are typically used when the researcher needs to address competing interventions in different reviews to report and highlight results. They typically include quality assessment of the studies within the reviews or of the reviews themselves and are often presented graphically or tabularly with some narrative aspects.
4. Scoping Reviews
A scoping review can be done in conjunction with any of the other review types on this list. Typically, a scoping review is done to determine the potential size and scope of literature available. A research team might conduct a scoping review to help develop, prioritize, and refine research priorities and inform future reviews or primary research. Scoping reviews are also often used to predict resource requirements (time and budget) to help define review protocols. Since scoping reviews are designed to determine the size and scope of literature available, quality assessment is not needed. Scoping reviews are presented in a tabular format with narrative.
5. Rapid Reviews
The rapid review is essentially a fast-tracked version of the systematic review. Rapid reviews are typically done when policymakers are working within a specific, tight timeframe and need quick turnaround. As a result, some critical systematic review steps are either modified or skipped entirely in a rapid review. For example, we might see less comprehensive search strategies, reduced use of grey literature (which can be challenging to find and process), more basic data extraction, and only simple quality appraisal. Results are often presented in a narrative and tabular format.
6. Qualitative Reviews
A qualitative review looks at themes and concepts across individual qualitative studies. Qualitative reviews may also be known as “meta synthesis” or “qualitative evidence synthesis.” Qualitative reviews employ quality assessment, but unlike systematic reviews, where the assessment is done to determine inclusion or exclusion, quality assessment in a qualitative review is used to mediate messages. Naturally, this type of review uses a narrative approach to presenting results, but tables and diagrams are also often used.
7. State-of-the-Art Reviews
While most systematic reviews include the entire scope of literature available on a particular topic, the state-of-the-art review generally focuses on recently published literature to assess current matters. A state-of-the-art review will often highlight new ideas or gaps in research with no official quality assessment. State-of-the-art is also a term used heavily in the medical device space. For researchers working on clinical evaluation reports (CERs), establishing “state-of-the-art” means describing “what is currently and generally considered standard of care, or best practice, for the medical condition or treatment for which the device is used.” Although state-of-the-art is typically considered just one section of the CER, it’s important to note that this type of review has an impact on all other aspects of the CER and if the state-of-the-art section fails, the entire report could also be invalidated. In general, they tend to report results using both a narrative and tabular component.
8. Mixed Study Reviews
When more than one research method is used, it’s known as a mixed study review. In most cases, a mixed study review involves combining multiple systematic review methods to synthesize different types of research. An example of this would be combining evidence from quantitative and qualitative studies. Another characteristic of a mixed study review is the use of appraisal tools or specific appraisal processes compared to other types of reviews. Since these reviews incorporate multiple different study types, the results presentation may include narrative, graphical, and tabular components.
Bonus: Meta Analysis
Although this isn’t technically a type of systematic review, we would be remiss to not mention meta analysis in this post. At one point some teams used the terms meta analysis and systematic review interchangeably. Meta analysis is now more commonly used to describe a quantitative synthesis method. Meta analysis is the process of statistically combining and comparing quantitative studies. However, in many research fields, meta analysis is no longer considered a type of review. Instead, researchers use meta analysis to describe the statistical technique used to combine results within a systematic review, but it can also be used in research that was not collected systematically.
Applying a systematic approach to research minimizes bias in study selection and synthesis to best inform decision-making. Additionally, applying systematic methods to research also improves the transparency and reproducibility of the research process, which is critical to maximize their utility.
Systematic review software like DistillerSR offers a highly configurable platform to conduct any type of systematic review. Regardless of your industry or the type of review you perform, DistillerSR enables teams to complete reviews efficiently and effectively according to their protocol. Want to see it in action? Request a free demo and see how DistillerSR can be easily configured to your preferred systematic review type.