Not all research questions lend themselves to systematic review. There are other review types that employ thorough, documented search strategies, but that aren't quite as intensive and time consuming as systematic reviews.
If you plan to conduct a review of the literature, it is important to determine which type of review is most appropriate for your purposes. Types of evidence synthesis include:
The purpose of a scoping review is to assess the potential size and scope of available literature for a research question. Scoping reviews can be useful if you have a broad question and would like to gain a better sense of major themes or topics. The end goal can be the scoping review itself, or you may be able to use the findings from a scoping review to create a more well-defined research topic for a systematic review.
A rapid review employs many of the same methods used for systematic reviews, but is usually completed in a shorter span of time. Due to time constraints the search strategy used may be less comprehensive than traditional systematic reviews. Grey literature might not be included either. This type of review is typically undergone for urgent clinical decisions or emerging needs.
An umbrella review allows the findings of reviews relevant to a review question to be compared and contrasted. An umbrella review's most characteristic feature is that this type of evidence synthesis only considers for inclusion the highest level of evidence, namely other systematic reviews and meta-analyses. Umbrella reviews provide a ready means for decision makers in healthcare to gain a clear understanding of a broad topic area.
A statistical analysis that combines or integrates the results of several independent clinical trials considered by the analyst to be ‘combinable
The rationale for a meta-analysis is that, by combining the samples of the individual studies, the overall sample size is increased, thereby improving the statistical power of the analysis as well as the precision of the estimates of treatment effects.
Review articles in the medical literature have traditionally been in the form of “narrative reviews” where experts in a particular field provide what is supposed to be a “summary of evidence” in that field. They are often not explicit about how they selected, assessed, and analysed the primary studies, thereby not allowing readers to assess potential bias in the review process. Narrative reviews are therefore often biased, and the recommendations made may be inappropriate.
A methodical and comprehensive literature synthesis focused on a well-formulated research question. Systematic reviews aim to identify and synthesize all of the scholarly research on a particular topic, including both published and unpublished studies. They are conducted in an unbiased, reproducible way to provide evidence for practice and policy-making and to identify gaps in research. They are much more time-intensive than traditional literature reviews and other evidence syntheses. They may involve a meta-analysis.