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Systematic Reviews: Grey Literature

A guide for those who want to do a Systematic Review search strategy mainly in the health sciences field

What is the Grey Literature

Grey Literature (GL) represents materials that cannot be found easily through conventional channels, "but which are frequently original and usually recent”: unpublished or published in obscure journals; unpublished trial data; invisible; fugitive; untouchables of the library world.

“produced at all levels of government bodies, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers'‘    Luxembourg Convention

Grey Literature includes:

  • hard to find studies, reports, or dissertations
  • conference and meeting proceedings/abstracts
  • governmental or private sector research
  • clinical trials - ongoing or unpublished
  • informal communications with experts
  • Technical or research reports from government agencies
  • Reports from scientific research groups
  • Dissertations and theses
  • Clinical practice guidelines not published in journals
  • Reports identified via Google searching

Conferences

Conference proceedings are difficult to find as they may be published in a book, journal or series, or may not be published at all. Societies/Associations usually publish abstracts from a conference, but posters are often not published.  Authors may "publish" their conference papers or posters on personal or institutional websites. So hand-search abstracts/conference proceedings appearing in journal supplements; also search Google (use conference|presentation|abstract as keyword), or use:

NIH Conference Proceedings Guide

Clinical Trials Registries

Clinical trials register clinical research studies that are recruiting and reports those that have been completed.  Since a majority of the trials are never published, it is important to search below sites if you're looking for clinical trial data:


Recommended reading: Medical and health-related trials registers and research registers maintained by Julie Glanville and Carol Lefebvre and hosted by the York Health Economics Health Consortium.  

Grey Literature Search Engines

Some of the most useful general grey literature search engines:

International Directory of Organizations in Grey Literature: organizations are listed by country.

Other Grey Literature Resources

Unbiased SR searches look for unpublished literature in a number of places, included meeting abstracts, white papers, clinical trial registries in addition to:

Theses or Dissertations

Citation Tracking can be either forward or backward. Forward Citation Tracking provides information on articles cited by other authors and can be searched using:

Whereas the backward citation tracking or snowballing involves checking the reference lists of key articles, SR or book chapters.

Pearl Growing or Pearl Building is when one identifies additional relevant controlled vocabulary and keywords from relevant documents.

Hand searching a manual method of searching through select journals from cover to cover to find studies that might have been missed through indexing.  This is done because not all journals are indexed, and some are indexed selectively. Select journals that contain abstracts of conference proceedings and are available to you. Resources to identify journals for hand searching include:

Miscellaneous options:

  • Search bibliographies of similar systematic reviews, meta-analysis and guidelines;
  •  Organizations producing systematic reviews are most likely to have or know of unpublished reviews;
  •  Identify key researchers and contact them:
    • Drug or device manufacturer
    • Experts in the field