What is grey literature?

When we think of scientific research, the first thing that usually comes to mind is articles published in academic journals, peer reviewed and indexed in well-known databases. Yet there is another rich source of information that often goes unnoticed: grey literature.

Definition

According to the International Conference on Grey Literature (2004), grey literature is:

“Information produced at all levels of government, academia, business, and industry, in electronic or print formats, not controlled by commercial publishers, meaning that publishing is not the primary activity of the organization producing it.”

In other words, these are publicly available sources of information produced outside traditional publishing channels, such as scientific journals.

Why “grey”?

The name reflects its position in a middle ground: between material openly published and indexed in formal databases (sometimes called white literature) and material that is inaccessible or absent from the public domain (dark literature).

Main types of documents

Grey literature includes a wide range of documents. The table below outlines the most common types:

Advantages

• Broadens the scope of research by including reports, guidelines, and data not published in journals;
• Reduces publication bias by including negative or neutral findings;
• Provides access to up-to-date information;
• Provides local and contextual evidence for public policies and regional practice;
• Helps identify gaps and trends in scoping reviews;
• Offers a variety of formats, including raw data, guides, and technical opinions.

Disadvantages

• Most documents have not been peer reviewed;
• Information can be difficult to locate, access, and standardize;
• The same material may appear in different repositories;
• Older documents may be out of date;
• Diverse formats make systematic organization and comparison more difficult;
• Additional critical appraisal is needed, especially in systematic reviews (see the next section).

What about information quality?

Most grey literature documents do not undergo peer review, so their methodological quality, scientific rigor, and data reliability can vary.

This calls for particular care, especially in relation to the type of review you are conducting:

In systematic reviews:
Grey literature can be included, but it requires rigorous critical appraisal using specific tools to assess quality and risk of bias, such as GRADE or AMSTAR 2. Systematic reviews aim to synthesize the best available scientific evidence, and the strength of that evidence depends directly on the quality of the studies included.

In scoping reviews:
The aim is to map the available information on a topic, rather than necessarily assess the methodological quality of every source. Grey literature is particularly useful here because it helps reveal gaps, trends, and a wider variety of data, including material that never reaches formal scientific journals. It also helps reduce publication bias, since studies with negative or neutral findings often remain unpublished in journals.

Where can you find it?

1. Academic search engines and aggregators

2. Repositories for theses, dissertations, and institutional research outputs

3. Government and international organization platforms

4. Open access books and technical publications

5. Platforms for preprints, protocols, and research data (datasets)

6. Manual searching

Not all grey literature is indexed in databases or search engines. You may therefore need to visit institutional websites directly to find relevant documents. Examples include:

• Government agencies, such as the Centers for Disease Control and Prevention (CDC), Brazil’s National Cancer Institute (Inca), or the Food and Drug Administration (FDA);
• Professional councils and associations, such as Brazil’s Federal Nursing Council (Cofen), the American Nurses Association (ANA), or the Royal College of Nursing (RCN);
• Institutional repositories and digital libraries, such as Universidade Federal Fluminense’s institutional repository (RiUFF), the Oswaldo Cruz Foundation’s Arca repository (Fiocruz), or DSpace at the Massachusetts Institute of Technology (MIT);
• Nongovernmental organizations, such as Médecins Sans Frontières and Save the Children;
• International organizations, such as the United Nations Children’s Fund (UNICEF) and the Pan American Health Organization (PAHO).

Search directly on these websites or in their dedicated sections, using the internal search tool or browsing areas such as publications, digital libraries, technical documents, and reports.

References

GREYNET INTERNATIONAL. Grey Literature Network Service. Amsterdam: GreyNet International, c2025. Available from: https://www.greynet.org/greysourceindex/documenttypes.html. Accessed July 17, 2025.

MCLEOD, M. Grey Literature. Canada: Carleton University, 2025. Available from: https://library.carleton.ca/guides/subject/grey-literature. Accessed July 17, 2025.

RILEY, C. Grey literature: What it is & how to find it. Canada: Simon Fraser University Library, 2025. Available from: https://www.lib.sfu.ca/help/research-assistance/format-type/grey-literature. Accessed July 17, 2025.

UNIVERSITY OF EDINBURGH. Grey literature. Scotland: University of Edinburgh, 2024. Available from: https://library.ed.ac.uk/finding-resources/library-databases/databases-subject-a-z/grey-literature. Accessed July 17, 2025.