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- Grey literature
What is grey literature?
"The term ‘grey literature’ is often used to refer to reports published outside of traditional commercial publishing. Review authors should generally search sources such as dissertations and conference abstracts" (Higgins et al. 2019, section 4.3.5). Grey literature may include research reports, conference papers, dissertations and theses, clinical trials, government documents, census data, standards, patents, and other research outputs.
Grey literature has traditionally been considered somewhat difficult to locate, but it is important to consult these unpublished studies to reduce the risk of publication bias in results.
Reference: Higgins, J. (2019) 'Searching for studies' in Higgins, J. and Thomas, J., eds., Cochrane handbook for systematic reviews of interventions [online], Version 6, available: http://handbook.cochrane.org/ [accessed 27 May 2020].
Why is grey literature important?
Grey literature is used for many reasons. These may depend on your field of research but some of the key reasons include:
- It introduces alternate viewpoints
- It is the only source of information- e.g. research data, letters and unpublished studies
- It provides first hand accounts of events - e.g. research data, diaries
- It overcomes, or minimises, reporting or publication biases; grey literature is more likely to include negative results - e.g. clinical trials
- It provides new information not yet published in traditional sources - e.g. conference papers
- It provides more local information - e.g. government reports, local collections
- It is a source of raw data such as data sets and statistics
How to search Grey literature for systematic reviews
Preprint is a term that tends to mean a version of a manuscript that is self-archived and shared publicly before publication in a scholarly journal - often the version prior to peer review. You may wish to include searches for preprints in your systematic review.
Web of Science Preprint Citation Index (1991-present) Preprint Citation Index provides nearly two million preprints from the arXiv, bioRxiv, chemRxiv, medRxiv, and Preprints.org repositories. Papers from more than a dozen additional repositories will be added throughout 2023.
In some literature databases in a specific field, e.g. PsycINFO, you can search for preprints. See the video How to Find Preprint Articles in PsycINFO
Preprints can also be found in the repsository of institutions, e.g. Research portal University of Groningen
Orther sources to find preprints:
Research reports can be found through a web browser, for example Google Scholar, or via the website of policy institutes and organizations.
Other sources to find research reports:
OpenAIRE Dutch research output
JSTOR More than 39,000 research reports from over 140 policy institutes around the world
BASE One of the world's most voluminous search engines especially for academic web resources. You can limit your search to a specific document type, e.g. conference object or report.
In some literature databases, e.g. Web of Science and Scopus, you can search for conference proceedings.
Many conference proceedings are open access and can be found through a web browser, for example Google Scholar, or via the website of institutes and organizations.
BASE One of the world's most voluminous search engines especially for academic web resources. You can limit your search to a specific document type, e.g. conference object or report.
Dissertations University of Groningen
Thesis research portal
European dissertations
DART-Europe E-theses Portal Open access research theses from 579 Universities in 29 European countries. Filtering by country or university is possible.
International dissertations
Open dissertations
ProQuest Dissertations & Theses A & I (Abstracts and Indexes)
Networked Digital Library of Theses and Dissertations (NDLTD) Provides Resources to find available open-access electronic theses and dissertations.
Each country has its own way of making government documentation available.
Netherlands
Overheidsdocumentatie
United States of America
US Government information
United Kingdom
UK official documents
European Union
EU Publications
Reporting grey literature searching
When you are writing up your review remember to report on any grey literature searching you have done. Please note that reproducibility is harder to achieve for grey literature searches, due to the frequency of changes to websites and search engines.
Describe any online or print source purposefully searched or browsed (e.g., tables of contents, print conference proceedings, web sites) and how this was done.
Examples:
- “We also searched the grey literature using the search string: "public attitudes" AND "sharing" AND "health data" on Google (in June 2021). The first 20 results were selected and screened.”
- “The grey literature search was conducted in October 2021 and included targeted, iterative hand searching of 22 government and/or research organization websites that were suggested during the expert consultation and are listed in S1 Protocol. Twenty two additional citations were added to the review from the grey literature search.”
More information can be found in the Prisma-statement extension for searching.