There are many cases where a systematic review is not appropriate. Some notable reasons to choose another review type are:
There are many other types of reviews that may be appropriate in such cases, or simply that serve other, distinct and important purposes. Six major review types (including systematic reviews) are outlined below. For a full explanation of the many types of reviews, please read the Grant & Booth (2009) article, A typology of reviews: An analysis of 14 review types and associated methodologies.
"Review Methodologies Decision Tree" by Cornell University Library is licensed under CC BY 4.0.
Narrative Reviews:
Narrative reviews are also known as literature reviews. They differ from the 'literature review' optimally included as part of a published, primary research study in that the literature review forms the entire paper and provides a more thorough overview of the topic of interest. Narrative reviews are a subjective overview of a topic within recent literature and can help to identify major omissions and gaps in existing research.
Integrative Reviews:
Integrative reviews provide a comprehensive overview of the central issues in a topic area. They synthesize both experimental and non-experimental research, pulling together studies with diverse methodologies to provide a broad but thorough overview of the work done in a subject area (Whittemore & Knafl, 2005). Above and beyond what is done in a narrative review, integrative reviews generally include an evaluation of the strength of evidence for the studies described in the review.
Scoping Reviews:
Scoping reviews provide an assessment of available research in a topic area, with the aim of identifying the nature and extent of research evidence available. Scoping reviews can inform whether or not a full systematic review is needed and feasible. The methods used for scoping reviews are systematic, transparent and replicable. The search methods, selection process, and assessment methods for included studies ought to be reported for a high-quality scoping review. Scoping reviews typically take longer to complete than both narrative reviews and integrative reviews.
Systematized Reviews:
Systematized reviews include some of the elements of a full, systematic review, but not all. Typically, a systematized review will include a comprehensive and reproducible search strategy, a screening process similar to what is used in a systematic review, and a table of included studies with information about the population, methods, and outcomes for each study. Unlike systematic reviews, systematized reviews normally have just one reviewer, restrict the search to one or two databases, and may lack formalized risk of bias assessments. Systematized reviews are often the foundation for a dissertation or a fully funded research project.
Systematic Reviews:
Systematic reviews require a clearly formulated and appropriately focused research question so that the studies included can be meaningfully compared and synthesized. As an example of how an overly broad question might cause problems, if a researcher undertook a systematic review on the effect of bariatric surgery on interpersonal relationships, the researcher would have studies with many different outcomes and populations of interest. This would make synthesis a difficult and subjective process. Systematic reviews also require a team, a registered protocol, a high quality search across multiple databases, consistently applied selection and appraisal methods, and an overall approach intended to limit bias, including blinding during the selection and appraisal processes.
Meta-Analyses:
A meta-analysis is "the statistical analysis of a large collection of results from individual studies for the purpose of integrating the findings" (Glass, 1976, p. 3). Meta-analyses combine the results of individual quantitative studies to investigate variations between the results of each included study. In order for a meta-analysis to be feasible, all included studies must be sufficiently similar. For example, if a meta-analysis is focused on overweight individuals as the population, and some included studies used BMI to classify overweight status, whereas other studies used waist-to-hip ratio and still other studies used skinfold measurements, the classification of overweight status across studies might be sufficiently different to cause problems with combining the results. A trustworthy meta-analysis ought to be based on a rigorously conducted systematic review. Although the published literature contains meta-analyses not based on systematic reviews, the lack of a thorough search compromises the representativeness of the data and that may significantly skew the results of the meta-analysis.
Glass, G. V. (1976). Primary, secondary, and meta-analysis of research. Educational researcher, 5(10), 3-8.
Grant, M. J., & Booth, A. (2009). A typology of reviews: An analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26(2), 91–108. doi:10.1111/j.1471-1842.2009.00848.x
Whittemore, R., & Knafl, K. (2005). The integrative review: updated methodology. Journal of Advanced Nursing, 52(5), 546–553. http://doi.org/10.1111/j.1365-2648.2005.03621.x