Grey literature is a type of scientific information that is not formally published as articles in scholarly journals. It could be reports, dissertations, manuscripts, clinical guidelines, produced by governments, universities and private companies.
Why search and use grey literature?
Grey literature can be an important complement because it allows you to find information that is not available elsewhere. Grey literature can provide different perspectives, and can be used to highlight and discuss a topic in a broader context. Grey material can also provide a more detailed description of a particular method or technique, for example.
You may also find more up-to-date information in grey literature compared to other sources. Grey literature usually does not undergo the same peer review as scientific publications, and therefore does not take as long to publish. It may also include information that has not yet been accepted for publication in a scientific journal. Because the grey literature does not follow the same standardized reporting structure as scientifically published articles, its quality and reliability may vary.
Grey literature in systematic reviews
Searching for and including grey literature in systematic reviews is mentioned and recommended in several different handbooks and checklists, including the Cochrane Handbook and PRISMA. It is also common to include grey literature in other types of reviews, such as scoping reviews.
One reason for including grey literature in systematic reviews is to reduce the risk of publication bias. Publication bias means that studies with positive results are more likely to be published than studies with negative results, which can lead to an overrepresentation of studies with positive results in the published scientific literature.
Searching for grey literature
To find grey literature, you may need to use several different approaches and sources. Searching in Google Scholar or other search engines can be a good start, but to avoid missing relevant literature, other approaches are also recommended. How many and which sources to use depends on the amount of time and resources you have available - searching for grey literature can be time-consuming - and on the type of grey material you want to find.
In general, you need to use a simplified search strategy when searching for grey literature compared to searching in regular databases. In some sources, you need to search with only one or a few keywords.
Many sources where you find grey literature have limited possibilities when it comes to exporting the results to, for example, a reference management program. You may therefore need to make the selection right during the search process.
If you want to learn more
Summarized Research in Information Retrieval for HTA (SuRe Info) has an informative and recently updated chapter on grey literature.
Searching the Literature: A Guide to Comprehensive Searching in the Health Sciences - Grey literature detailed guide from the University of Toronto Libraries. Includes not only tips on resources to search, but also suggested approaches and documentation.
Where to search for grey literature
Below are some resources and search services that can be useful for finding different types of grey material.
- Google Scholar: Google's search engine for scientific literature. Google Scholar searches the full text of articles, unlike many databases. However, one tip is to try limiting the search to the title field (can be done by clicking on advanced search), which sometimes gives a more relevant search result. See this guide for general search tips. To export many hits at once (up to 1000) from Google Scholar, you can use Harzings Publish or Perish.
- BASE (Bielefeld Academic Search Engine): A search engine for scientific literature, containing over 300 million documents from 10,000 different sources. Good possibilities to limit the search in terms of origin, language, material type and subject. Operated by the Bielefeld University Library in Bielefeld, Germany.
- Semantic Scholar: Semantic Scholar is a freely available AI-powered research tool, developed by the Allen Institute for AI in the US. It contains over 200 million academic publications. Semantic Scholar uses natural language processing and machine learning techniques to provide summaries of scientific articles, and also includes several features for exploring citations.
- Lens.org: Lens is a freely available search engine for scientific publications and patents. It contains more than 200 million scientific publications and more than 120 million patents. Lens retrieves bibliometric data from other sources such as PubMed, Crossref and OpenAlex. It is possible to perform more advanced searches and there are several ways to filter your search. Operated by Cambia, a non-profit company based in Australia.
- Dimensions AI: Dimensions AI is a database that includes millions of research publications and links them to over 1.7 billion citations, research grants, datasets, clinical trials, patents and policy documents. Part of Dimensions AI is freely available, but full access requires a subscription to the database. Produced by Digital Science, a technology company based in London.
- OAIster - OAister is a search tool that collects and aggregates metadata from various databases and digital collections for open access resources. OAister is produced by OCLC (Online Computer Library Center) and aims to increase access to open access materials. Contains over 50 million documents.
Ongoing clinical trials and clinical studies can be found in clinical trial registries. The information in the registries is updated when the study is completed and published. RCTs and quasi-RCTs from ICTRP and ClinicalTrials.gov are included in the Cochrane Central Register of Controlled Trials (CENTRAL). However, it is recommended to search directly in ICTRP and ClinicalTrials.gov to ensure that all relevant ongoing clinical trials are found.
- The World Health Organization International Clinical Trials Registry Platform Search Portal (WHO ICTRP): retrieves information from registries worldwide, and aims to provide consolidated access to information on ongoing and completed clinical trials.
- ClinicalTrials.gov: is a database of clinical research studies with information on their results. Includes studies from over 200 countries. Also searchable via ICTRP.
- Clinical Trials Information System (CTIS): As of 31 January 2023, all applications for clinical trials in the European Union (EU) must be submitted through the Clinical Trials Information System (CTIS). The system was launched in January 2022 and currently contains a limited amount of clinical trials but will gradually grow. Also searchable via ICTRP.
- The EU Clinical Trials Register (EUCTR): contains information on interventional clinical trials of pharmaceuticals conducted in the European Union (EU) or the European Economic Area (EEA) and initiated after May 1, 2004. The database mainly covers trials initiated before 31 January 2023. Also searchable via ICTRP.
- HiKS - Hitta Kliniska Studier: is an information service where you can find approved clinical trials in Sweden. Responsible for the service is Lif, the trade organization for research-based pharmaceutical companies.
To find reports and publications from organizations, authorities and governments, you often need to search directly on the website of the authority or organization. You can also find these kinds of publications by using some of the resources under the heading "General/search engines" above.
- Google site search: Google Site Search is a feature that allows you to search for content on a specific website using the Google search engine. It can be useful if the organization or authority does not have its own good search functionality on the site. Example: search in Google for site:ki.se "dyslexia" to find all web pages under the domain ki.se where the word "dyslexia" appears.
- Greylitsearcher: An R package and an R Shiny app that uses Google site search to enable searching multiple web pages simultaneously.
- Policy Commons: Policy Commons is a resource where you can find publications from think tanks, government agencies and organizations. It contains policy reports, administrative documents, books, reports, press releases, brochures, etc. There are good opportunities to limit your search in different ways and to make more advanced searches. Policy Commons is operated by Coherent Digital.
- International HTA database: International HTA database where you can find both ongoing and published assessments by HTA organizations worldwide, including the Swedish SBU.
- List of Foreign Governments: Here you will find links to the websites of governments and authorities in different countries. The list is created by the Northwestern University Library.
Conference publications can be found in some databases, and also via several of the resources under the heading "General/search engines" above. It is often possible to filter your search by conference proceedings or conference papers. You can also add conferences as a search term. Conferences can also be referred to as meetings, symposia, seminars, colloquia, congresses, workshops, or conventions.
- The library's search tool reSEARCH: Under "Popular limitations" to the right of the results list, you can limit your search to "Conference proceeding".
- SwePub: In SwePub you will find different kinds of academic publications, including conference publications, from Swedish universities. Filter your search by conference paper.
- Web of Science: contains an index of citations of conferences in sciences, social sciences and humanities from 1990 until today.
- Embase: contains nearly 5 million conference abstracts from over 12,000 different conferences. The Elsevier website provides a list of the conferences included.
You can find theses via several of the resources under the heading "General/search engines" above, and also in some databases, such as CINAHL. There are also a number of good resources to search in if you want to find theses:
- SwePub: In SwePub you will find different kinds of academic publications, including conference publications, from Swedish universities. Filter your search by dissertations.
- Dissertations and Theses (The Sciences and Engineering Collection): A database that provides access to an extensive collection of interdisciplinary dissertations from around the world. Includes full text for the majority of dissertations from 1997 onwards. The database is produced by ProQuest and is not freely available, only accessed via KI.
- DART-Europe: Search engine for European doctoral theses from 570 universities in 28 countries. Operated in a collaboration between European universities.
- WorldCat: WorldCat is a global library catalog where you can search for publications cataloged in libraries around the world. In the advanced search form, in the drop-down menu under "Format", you can limit your search to "thesis/dissertation" .
Preprints can now be found in several databases. You can also search for preprints directly via various preprint archives, but the search options there are often more limited than in databases.
- Embase: since November 2021 Embase contains preprints from bioRxiv and medRxiv.
- PubMed: since January 2023 PPubMed contains preprints from NIH-funded research, and preprints from bioRxiv, medRxiv, arXiv and Research Square.
- Europe PMC: contains preprints from 2018 onwards, from several different preprint archives.
- bioRxiv: preprint archive in biology
- arXiv: preprint archive covering many different subject areas
- medRxiv: preprint archive in health sciences
- psyArXiv: preprint archive in psychology
Practical guidelines are sometimes published in scientific journals and can be found via databases such as PubMed. You can also find practical guidelines via several of the resources under the heading 'General/search engines' above.
- National guidelines of the National Board of Health and Welfare: The National Board of Health and Welfare is the authority in Sweden responsible for producing national guidelines for health care, dental care and social services.
- National clinical knowledge support: National system for knowledge-driven management within Swedish healthcare. Includes care programs and guidelines.
- Databases with peer-reviewed summaries of current evidence, such as UpToDate and DynaMed. Medical libraries often provide these types of databases.
- TRIP (Turning Research Into Practice): Search engine focusing on evidence-based medicine. Easy to filter the search results on practice guidelines, and other types of publications. TRIP has a freely available version but a subscription is needed to use the database fully and access all the materials.
- Specialist associations: You can often find guidelines and recommendations in specific areas from various medical specialist associations and organizations. Some examples are: European Society of Cardiology (ESC), American College of Physicians (ACP), American Psychiatric Association (APA), National Comprehensive Cancer Network (NCCN).
- Guidelines International Network (GIN): GIN is a global network organization that works to bring together and promote the development of clinical guidelines. The website contains a comprehensive collection of international guidelines in various medical fields.
- National Institute for Health and Care Excellence (NICE): NICE is a UK organization that produces guidelines, recommendations and guidance in the health sector.
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