Dear Editor,
A systematic review is considered as the strongest evidence and occupies the top place in the research pyramid. A researcher involved in conducting a systematic review and/or meta-analysis (SRMA) needs to perform a literature review that involves a search for articles with relevant keywords in various databases. The popular and recommended databases are PubMed/Medline, EMBASE (Excerpta Medical Database), SCOPUS, CENTRAL (The Cochrane Central Register of Controlled Trials), CINAHL (The Cumulative Index to Nursing and Allied Health), and Web of Science.[1]
A meticulous SRMA also involves something which is known as grey literature which refers to research that is produced on all levels of government, academics, business, and industry in print and electronic formats, but not controlled by commercial publishers.[2] Grey literature is not indexed by major databases but can be searched through various search engines. Grey literature basically comprises of reports, conference abstracts and posters, theses, dissertations, and white papers. In 2011, the Institute of Medicine suggested inclusion of grey literature search while performing any SRMA.[3]
The reason for performing a grey literature search is to reduce publication bias (by including data which is not peer reviewed or in an indexed database probably because of negative findings). Grey literature search also helps to gather data which is not available commercially so as to comprehensively review and analyze a research question and for a more precise effect size estimate. Experts feel that excluding grey literature from analyses could compromise the validity and reliability of meta-analyses and the specificity of systematic reviews.[4] McAuley et al.[5] found that 33% of meta-analyses reviewed included grey literature, accounting for 4.5% to 75% of studies in meta-analysis. The Cochrane Handbook for Systematic Reviews of Interventions also mentions that a failure to identify and analyze studies from various grey literature search could impact the results of an SRMA.[6]
There are several ways of searching documents available in the grey literature. The following are the websites: https://www.cadth.ca/grey-matters-practical-tool-searching-health-related-grey-literature, https://opengrey.eu/, https://www.nyam.org/library/collections-and-resources/grey-literature-report/, https://www.tripdatabase.com/, https://www.oclc.org/en/oaister.html, http://mednar.com/. The grey literature can also be searched on clinical trial registries (http://clinicaltrials.gov/), patent databases, company and industry repositories, digital archives (https://www.fda.gov/), clinical practice guidelines of various regions, factsheets, clinical communications, websites of various organizations and institutes, and newspapers.[7] As there are no recommended strategies for performing a grey literature search, it could turn out to be time consuming and at times frustrating if the search does not yield anything definitive. Taking the help of a librarian is recommended for a grey literature search.
In conclusion, searching grey literature and mentioning the search strategies are important while performing SRMA. However, the search strategies are heterogenous, are not streamlined, and take a lot of time. Nevertheless, searching it is essential for arriving at a definitive conclusion after performing a comprehensive SRMA.
Contribution details
Abhijit Nair: This author helped in concept, design, definition of intellectual content, literature search, data acquisition, data analysis, statistical analysis, manuscript preparation, manuscript review.
Nitin Kumar Borkar: This author helped in definition of intellectual content, literature search, data acquisition, manuscript review.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
1. Bramer WM, Rethlefsen ML, Kleijnen J, Franco OH. Optimal database combinations for literature searches in systematic reviews: A prospective exploratory study Syst Rev. 2017;6:245
2. Citrome L. Beyond PubMed: Searching the “grey literature” for clinical trial results Innov Clin Neurosci. 2014;11:42–6
3. Paez A. Gray literature: An important resource in systematic reviews J Evid Based Med. 2017;10:233–40
4. Benzies KM, Premji S, Hayden KA, Serrett K. State-of-the-evidence reviews: Advantages and challenges of including grey literature Worldviews Evid Based Nurs. 2006;3:55–61
5. McAuley L, Pham B, Tugwell P, Moher D. Does the inclusion of grey literature influence estimates of intervention effectiveness reported in meta-analyses? Lancet. 2000;356:1228–31
6. Higgins J, Green S Cochrane Handbook for Systematic Reviews of Interventions. 20115.1.0 ed Chichester, United Kingdom The Cochrane Collaboration
7. Adams J, Hillier-Brown FC, Moore HJ, Lake AA, Araujo-Soares V, White M, et al Searching and synthesising ‘grey literature’ and ‘grey information’ in public health: Critical reflections on three case studies Syst Rev. 2016;5:164
