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Grey Literature

What, Why, Where & How of Gray Literature

Gray Literature Sources

Source: Cochrane Interactive Learning: Gray Literature


Trials registers

The two key registers you should start with are and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP).

The ICTRP enables searching across multiple national registers. is an increasingly important international register for systematic reviews. It can be searched through the ICTRP, but its more advanced search features make an additional, direct search of this register worthwhile to find the largest possible number of relevant studies.

Not only do trials registers provide access to ongoing, unpublished studies, but they also provides access to a growing repository of information on completed studies, many of which have results posted on the site (irrespective of the publication status of the study). 

Clinical study reports

Clinical study reports (CSRs) are the reports produced, for example, on the studies conducted by drug manufacturers. These and other regulatory documents (those submitted to governments for approval or monitoring of new drugs or devices) can be used as a source of study data for systematic reviews, although the implications and methodology for using these sources is still under development. The reports are often extremely large, difficult to obtain, and may contain incomplete information, so seek advice if you wish to include these reports in your review.

Studies identified by contacting colleagues

You might also identify ongoing and/or unpublished studies by contacting individuals or organizations that you know are working in the field. Colleagues, including the authors of the studies included in your review, can be an important source of information about published or unpublished studies. Informal channels of communication can sometimes be the only means of identifying unpublished data.

Dissertations and theses

Dissertations and theses are normally not indexed in general bibliographic databases such as MEDLINE or Embase, but there are exceptions, such as CINAHL, where nursing, physiotherapy, and occupational health dissertations are indexed, and PsycINFO, where dissertations in psychiatry and psychology are indexed. To identify relevant studies published in dissertations or theses, it is advisable to search specific dissertation sources. You can find useful links in the Resources.

Conference abstracts and proceedings

Conference abstracts and proceedings are a particularly important source of grey literature. Some of them are indexed in bibliographic databases such as Embase, although not in MEDLINE, and may also be found on websites and in print. You can find useful links in the Resources. They are important to consider as research has found that over half of trials reported at conferences never reach full publication. In addition, conference proceedings are a good source to track disagreements between the original conference abstract and the full published report.

Source: Cochrane Interactive Learning: Gray Literature

  • CDC Stacks: free, digital archive of scientific research and literature produced by the Centers for Disease Control and Prevention (CDC). Composed of curated collections tailored for public health research needs. Access to current CDC research and literature such as the Open Access Collection. In addition, CDC Stacks offers a historical perspective that was previously not available, such as the first 30 volumes of the Morbidity and Mortality Weekly Report.

  • medRxivmedRxiv (pronounced "med-archive") is a free online archive and distribution server for complete but unpublished manuscripts (preprints) in the medical, clinical, and related health sciences. Preprints are preliminary reports of work that have not been certified by peer review. They should not be relied on to guide clinical practice or health-related behavior.

  • NY Academy of Medicine: Provides access to the NY Academy of Medicine "Grey Literature Report," a bimonthly alerting service.

  • Research Portfolio Online Reporting Tools, NIH (RePORT)  

    Reports, Data, and Analyses of NIH Research Activities

  • The OAIster® database  

  • Registry of Open Access Repositories (ROAR)  

  • OpenDOAR  

    OpenDOAR is an authoritative directory of academic open access repositories. Each OpenDOAR repository has been visited by project staff to check the information that is recorded here. This in-depth approach does not rely on automated analysis and gives a quality-controlled list of repositories.

  • OpenGrey 
    System for Information on Grey Literature in Europe, open access to 700.000 bibliographical references of grey literature (paper) produced in Europe and allows you to export records and locate the documents. Examples of grey (gray) literature include technical or research reports, doctoral dissertations, some conference papers, some official publications, and other types of grey literature.

    OpenGrey covers Science, Technology, Biomedical Science, Economics, Social Science and Humanities. The site includes preprints from the GL conferences (GreyNet International) in full text.

  • Organization Web Sites A - Z (from  

    Links to organizations, which may publish a variety of reports, white papers, pamphlets, data, presentations, posters, etc.

Mednar is a free, medically-focused deep web search engine that uses Explorit Everywhere!, an advanced search technology by Deep Web Technologies. As an alternative to Google, Mednar accelerates your research with a search of authoritative public and deep web resources, returning the most relevant results to one easily navigable page. Unlike Google, a Mednar search occurs in real-time, retrieving relevant medical information as if you were going to each individual website yourself.

NIH Gray Literature Databases

  •  HSRProj  Searchable information about health services research projects' research-in-progress before results are available in a published format.

  • Virginia Henderson Global Nursing e-Repository

    • A database of nursing research abstracts posted in the Registry of Nursing Research, over 11,000 English-language studies contributed by nurse researchers internationally. Indexed by variables or phenomenon of study as well as by author, title, kw provided by researchers. Includes study abstracts and conference abstracts.

  • Research Portfolio Online Reporting Tools, NIH (RePORT)  

    • Reports, Data, and Analyses of NIH Research Activities


    • Registry of federally and privately supported clinical trials conducted in the U.S and around the world