Finding grey literature, searching it systematically and documenting your searches is time consuming and challenging.
See the Grey literature library guide for further information on searching specific grey literature sources to identify relevant material, as well as using search engines such as Google/Google Scholar.
See the Moodle book MNHS: Systematically searching the grey literature for a comprehensive module on grey literature for systematic reviews.
Systematic reviews routinely search trials registers as a means of identifying additional unpublished and ongoing clinical trials and reducing the risk of reporting biases.
No single registry contains all studies. The Cochrane Handbook indicates that at a minimum the ICTRP (WHO International Clinical Trials Registry Platform, a meta-registry containing 17 registers) and ClinicalTrials.gov should be searched.
Cochrane Handbook 4.3.3 Trials registers and trials results registers #section-4-3-3.
Although there are many other trials registers, ClinicalTrials.gov and the WHO International Clinical Trials Registry Platform (ICTRP) portal are considered to be the most important for searching to identify studies for a systematic review (Pansieri et al 2017). Research has shown that even though ClinicalTrials.gov is included in the WHO ICTRP Search Portal, not all ClinicalTrials.gov records can be successfully retrieved via searches of the ICTRP Search Portal (Glanville et al 2014, Knelangen et al 2018).
Both of these important sources are contained in Cochrane CENTRAL (Cochrane Central Register of Controlled Trials) which is updated monthly and searchable via the Ovid platform. This comprehensiveness makes CENTRAL a popular choice for systematic reviews. As noted above, for Cochrane reviews they must search ClinicalTrials.gov and ICTRP instead for optimum sensitivity.
If using CENTRAL and including non-randomised study designs in your review, you should search ClinicalTrials.gov as well, as CENTRAL only indexes RCTs/quasi-RCTs from this source.
You might also include a national trials register (or another relevant register) separately in order to be rigorous (to overcome limited search functionality and lag in updates) or inclusive for your context. See the full curated list of registers.
Preprints play a significant role in systematic reviews by providing access to early and unpublished research findings. A preprint refers to a version of a research paper that has not undergone formal peer review but is made publicly available online. Here are some ways in which preprints are significant in systematic reviews:
Timeliness: Preprints allow researchers to access the most current research findings before they undergo the lengthy peer review process and are formally published. This is especially crucial in rapidly evolving fields where up-to-date evidence is essential for systematic reviews.
Access to unpublished research: Preprints provide access to research that might not otherwise be available for inclusion in systematic reviews. Traditional publication processes can be time-consuming, leading to delays in disseminating research. Preprints offer an avenue for researchers to share their findings promptly, enabling systematic reviewers to access a broader range of evidence.
Reducing publication bias: Traditional publication processes tend to favor positive results, leading to publication bias, where studies with negative or inconclusive findings are less likely to be published. Preprints help mitigate this bias by making both positive and negative research findings available for inclusion in systematic reviews. By including preprints, systematic reviews can present a more comprehensive and unbiased synthesis of the available evidence.
Enhanced transparency: Preprints contribute to the transparency of the scientific process by making research findings openly accessible. Including preprints in systematic reviews allows reviewers and readers to scrutinize the research methods, data, and findings more comprehensively. This transparency promotes robust discussions and enables other researchers to replicate or build upon the work.
Identifying ongoing research: Preprints can help identify ongoing or forthcoming studies relevant to a systematic review. Systematic reviewers can monitor preprint repositories to identify new studies in progress, collaborate with researchers, or consider including these studies once they are formally published.
Despite their benefits, it is essential to recognize that preprints are preliminary findings and have not undergone rigorous peer review. Including preprints in systematic reviews requires careful evaluation of the study quality and consideration of potential biases. Systematic reviewers should acknowledge the limitations and potential risks associated with relying on preprints in their analyses and interpretations.
Handsearching (manual searching)
While manual searching can still mean actually hand-searching print copies of journals for relevant studies, it more often involves browsing the online table-of-contents of relevant issues, or reference lists of relevant papers.
In systematic reviews, manual searching is considered an important method of uncovering papers that may not have been picked up in your database searches. This can include studies in journals not indexed by core databases, or papers not retrieved by your search strategy due to being poorly described or incorrectly indexed.
Citation searching
Citation searching comes under the umbrella of manual searching and is a search method that can be done forward or backward in time:
The main citation databases are Scopus, Web of Science and Google Scholar.