What is Grey Literature?

When we talk about scientific research, the first image that comes to mind is usually that of articles published in academic journals, peer-reviewed, and indexed in renowned databases. However, there is another incredibly rich universe that often goes unnoticed: grey literature.

Definition

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

"Information produced on all levels of government, academia, business and industry in electronic and print formats, not controlled by commercial publishers, i.e., where publishing is not the primary activity of the producing body."

In other words, they are informational documents available to the public but outside of traditional publishing channels, such as scientific journals.

Why "Grey"?

It gets this name because it is situated in an intermediate zone: between what is openly published and indexed in formal databases (the so-called white literature) and what is inaccessible or non-existent in the public sphere (the dark literature).

Main types of documents

Grey literature encompasses a wide variety of documents. Below, we detail the most common types:

Advantages

• Expands the scope of research, including reports, guidelines, and data not published in journals;
• Reduces publication bias by including negative or neutral results;
• Ensures access to up-to-date information;
• Provides contextual and local evidence for public policies and regional practices;
• Helps to map gaps and trends in scoping reviews;
• Diversity of formats, including raw data, guides, and technical opinions.

Disadvantages

• Lack of peer review in most documents;
• Difficulty in locating, accessing, and standardizing information;
• Possible duplication in different repositories;
• Risk of outdatedness of old documents;
• Heterogeneity of formats makes systematization and comparison difficult;
• Need for extra critical appraisal, especially in systematic reviews (see next topic).

And what about the quality of information?

Most grey literature documents do not undergo peer review, which means there can be variations in methodological quality, scientific rigor, and data reliability.

This requires extra attention, especially depending on the type of review you intend to conduct:

In systematic reviews:
Grey literature can be included, but it must undergo a rigorous critical appraisal using specific tools for quality analysis and risk of bias, such as GRADE or AMSTAR 2. This is because systematic reviews aim to synthesize the best available scientific evidence, and the strength of this evidence depends directly on the quality of the included studies.

In scoping reviews:
The goal is to map what exists on a topic, not necessarily to assess the methodological quality of each source. In this context, grey literature is very welcome, as it allows for the identification of gaps, trends, and a greater diversity of data – including those that do not make it into formal scientific journals. It also helps to reduce publication bias, as many studies with negative or neutral results end up not being published in journals.

Where to find it?

1. Search engines and academic aggregators

2. Repositories of theses, dissertations, and institutional productions

3. Governmental and international organization platforms

4.Open access books and technical publications

5. Platforms for pre-prints, protocols, and research data (datasets)

6. Manual searches

Not all grey literature is indexed in databases or search engines. Therefore, it may be necessary to visit institutional websites directly to find relevant documents. Some examples include:

• Government agencies, such as the Centers for Disease Control and Prevention (CDC), the National Cancer Institute (Inca) in Brazil, or the Food and Drug Administration (FDA);
• Professional councils and associations, such as the Federal Council of Nursing (Cofen) in Brazil, the American Nurses Association (ANA), or the Royal College of Nursing (RCN);
• Institutional repositories and digital libraries, such as the Institutional Repository of the Fluminense Federal University (RiUFF), the Arca of the Oswaldo Cruz Foundation (Fiocruz), or the DSpace at the Massachusetts Institute of Technology (MIT);
• Non-governmental organizations, such as Doctors Without Borders and Save the Children;
• International organizations, such as the United Nations Children's Fund (UNICEF) and the Pan American Health Organization (PAHO).

These searches should be conducted directly on the website or in its specific areas, using the internal search engine or Browse sections such as publications, digital library, technical documents, or reports.

References

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

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