The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as: "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Grey literature is research which is either unpublished or published through channels other than commercial publishing. Most of it is not found in bibliographic databases. You should consider what type of literature you are interested in before you begin searching. Not all grey literature is relevant to all researchers and it will depend on the subject studied (clinical trials will not be relevant to research about the environment for example but research reports may well be relevant).
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Why should you use grey literature?
Although grey literature can be very valuable and useful for your projects, it has often been through less thorough processes of reviewing and quality assurance. It is therefore crucial to carefully consider whether the grey literature you are using is trustworthy, reliable and accurate.
One tool you can use to help with evaluating grey literature is the AACODS Checklist for appraising grey literature