There are many definitions of grey literature, but it is usually understood to mean literature that is not formally published in sources such as books or journal articles. Conference abstracts and other grey literature have been shown to be sources of approximately 10% of the studies referenced in Cochrane reviews (Mallett 2002). In a recently updated Cochrane methodology review, all five studies reviewed showed that published trials showed an overall greater treatment effect than grey literature trials (Hopewell 2007b). Thus, failure to identify trials reported in conference proceedings and other grey literature might affect the results of a systematic review.
Conference abstracts are a particularly important source of grey literature and are covered in Section 6.2.2.4.
EAGLE (the European Association for Grey Literature Exploitation), has closed the SIGLE (System for Information on Grey Literature) database, which was one of the most widely-used databases of grey literature. INIST in France (Institute for Scientific and Technical Information) has launched OpenSIGLE, which provides access to all the former SIGLE records, new data added by EAGLE members and information from Greynet..
The Healthcare Management Information Consortium (HMIC) database contains records from the Library & Information Services department of the Department of Health (DH) in England and the King's Fund Information & Library Service. It includes all DH publications including circulars and press releases. The King's Fund is an independent health charity that works to develop and improve management of health and social care services. The database is considered to be a good source of grey literature on topics such as health and community care management, organizational development, inequalities in health, user involvement, and race and health.
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The National Technical Information Service (NTIS) provides access to the results of both US and non-US government-sponsored research and can provide the full text of the technical report for most of the results retrieved. NTIS is free on the internet.
PsycEXTRA is a companion database to PsycINFO in psychology, behavioural science and health. It includes references from newsletters, magazines, newspapers, technical and annual reports, government reports and consumer brochures. PsycEXTRA is different from PsycINFO in its format, because it includes abstracts and citations plus full text for a major portion of the records. There is no coverage overlap with PsycINFO.