Grey Literature Searching - HLWIKI Canada (

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Last Update

  • Updated.jpg 18 June 2017


See also Evidence-based public health | Expert searching | Grey data ("hard to find" data) | Grey literature searching in medicine | Grey literature - part II | Snowballing


  • "Grey literature is defined as ... "information produced on all levels of government, academics, business and industry in electronic and print formats not controlled by commercial publishing" ie. where publishing is not the primary activity of the producing body." — ICGL Luxembourg definition, 1997. Expanded in New York, 2004
  • "...grey literature publications are non-conventional, fugitive and often ephemeral. They may include but are not necessarily limited to the following types of materials: reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports, memoranda, state-of-the art reports, market research reports, etc.), theses, conference proceedings, technical specifications and standards, non-commercial translations, bibliographies, technical and commercial documentation, and official documents not published commercially (primarily government reports and documents)". — Alberani, 1990

Teach yourself


  • Grey - or gray - literature is historically difficult to identify because:
  • 1) much of it is unindexed or unpublished (often both), and;
  • 2) it is often locked deep within the "hidden or invisible" web.

The "Grey Zone"

  • Grey area or Zone - think of the grey zone as an in-between metaphor

Definition: An ill-defined area that does not readily conform to an existing category or set of rules - Oxford English Dictionary

  • Grey literature is used to describe materials not published commercially or indexed by major databases.
  • GL may be of questionable relevance or quality but may still have an impact in research, teaching and learning.
  • GL is occasionally the only source of information for specific research questions. While some GL may be published eventually, and may be easier to find, sometimes it never is.
  • GL may not go through a peer review process, and its authority must be scrutinized.
  • "Fugitive", hidden, invisible or literature in the deep web may be on government sites, deep in archives, institutional repositories, theses databases, conference sites, associations.
  • Informal communication is changing the notion of grey literature which is expanding to include e-mails, faxes, blog postings, wikis, RSS feeds and podcasts.

What differentiates traditional GL from other published literature?

  • Historically, the publication of GL has not been considered part of traditional publishing channels and models
  • Producers of GL include research groups, non-profits, universities and government departments, to name a few
  • Not widely disseminated or promoted
  • Wide dissemination of published materials is the goal in traditional publishing; infrastructure exists to disseminate this material to make it visible
  • The web is changing dissemination and publishing processes. However, findability remains difficult

Some examples of non-traditional publishing

  • Some organizations create their own reports, studies, etc. This is increasingly true in web 2.0.
  • Think of health organizations that publish their own studies, such as the Canadian Cancer Society and the Heart & Stroke Foundation
  • Government publications, see Finding medical / health care statistics online
  • Librarians try to adopt pro-active approaches to finding locally-published materials, though web-based searching, self-archiving and open access
  • Specialized strategies are needed to facilitate identification and retrieval of grey literature
  • The field of GL has evolved into a world of its own with specific research methodologies, vocabularies, systems and solutions. Before exploring these methods, let's highlight some of the work that health librarians are doing in this area.

Library & information research into grey lit (GL)

  • Search engines help uncover a ton of grey literature. Banks says that the barriers to grey literature are coming down as a result of open access and search engines. Search expert and librarian, Gary Price, says that 'public information on the deep web is currently 400 to 550 times larger than the surface Web'. In light of institutional repositories and open archives the deep web is more accessible than ever but much continues to be locked away behind commercial (or password-protected) databases.
  • Health literature fares better than some areas. But conference proceedings, abstracts and government reports in the pre-digital era are difficult to locate. The web provides access to a trillion+ pages of content, but not all relevant information is digitized yet. Librarians should work towards improving access to older materials which are, mostly, grey literature.

Some places where grey literature is abundant

  • Work done at the PhD-level requires exhaustive searching for hard-to-find materials
  • Systematic reviews, clinical trials and in-house research that cover health and wellness issues
  • Most advanced research done at universities, medical schools and health organizations
  • Environmental organizations, engineering publications and newsletters, conservation resources
  • Geological and geophysical surveys, maps, fossil records, locations of minerals and ores
  • Grey literature in technical fields, aeronautics and engineering may include contractor and technical reports, product codes and standards, special publications, handbooks and patents
  • See also the HTAi Vortal developed by librarians

Copyright issues

Medical studies, missed studies, bias

Effective searching is a professional skill undertaken by librarians and information specialists. The aim of searching in the health sciences is to be as thorough as possible and to optimize recall with sufficient precision. Researchers doing systematic reviews (SRs) or meta-analysis should ensure that all relevant studies are found. The focus is on exhaustiveness and leaving no stone unturned. Reviews are useful tools for health professionals in view of the massive amount of biomedical information published worldwide as they are a useful distillation of evidence.

It is imperative that health librarians create search strategies and execute them accordingly. When a structured search is not performed, search results may be affected. Studies in other languages or those not indexed by major tools must also be located to avoid skewed results, and publication bias. In a recent study, it was estimated that an additional 29.2% items were found by using extended search methods in addition to mainstream sources. (see Savoie et al).

Some researchers suggest that SRs that include grey literature of uncertain quality may actually jeopardize the findings of reviews. This is where rigorous inclusion criteria can ensure that only truly relevant studies make it into the final analysis. It is a common occurrence that high numbers of studies retrieved by the librarian may not make it into the final review assessment.

In a 2005 study, researchers showed that there are consequences (and some risk) associated of generalizing the importance of grey literature in avoiding publication bias in the field of psychiatry. (Martin et al. European Psychiatry. Is grey literature essential for a better control of publication bias in psychiatry? An example from three meta-analyses of schizophrenia. 2005 Dec;20(8):550-3.

Major trends in GL

Open Access (OA) to materials and the creation of institutional repositories has revolutionized publishing and the work of providing access to published works. Despite pivotal information trends like these, some digital and print materials are still hard to find and obtain. All librarians and information specialists have personal stories about finding elusive conference proceedings, abstracts and reports. The digital age has not completely changed that - not yet in any case.

The emergence of search engines has helped to index (and make findable) a lot of GL. But searching carelessly with Google creates other problems for information specialists as important documents can easily be missed. Even though the Web is estimated to be 500 billion - perhaps as high as one trillion pages, its functionality as a search space is limited due to its methods of organization.

Computer algorithms help to improve search engines. Pagerank uses popularity as a means of ranking results with important items rising to the top. But by placing popular materials at the top of results, searchers will rarely beyond the first six or seven results.

As librarians know, relying on popular documents that rise in search results is not a recommended strategy. Important documents may be easily retrieved via search engines, but some GL may ultimately be hidden within results, down several pages or not visible at all due to a relative lack of popularity. (Other trends to watch: collaborative writing/publishing via Wikis and blikis.)

Indexes & Databases with Grey Lit

See also 189 relevant health databases


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