There's no single, agreed definition of grey literature. But that's part of the point of grey literature in a way. It's 'grey' because it occupies a 'grey area': an ambiguous state.
Grey literature is generally used as a catch-all term for any material which hasn't been subject to a formal publication process. Whereas a book or journal article might be edited, peer-reviewed and so on, grey literature is more likely to exist outside these commercial academic publishing processes.
Examples might include (but are no means limited to) reports, conference proceedings, dissertations, theses, presentations, government publications and policy papers. Some would even say that it covers everything except books and journal articles; in other words, a huge amount of material! This page itself might even be considered grey literature. We're getting very meta now...
Given all of that, grey literature could well be a physical item (and historically always would have been) but in our increasingly online world it's increasingly more likely to be something we'd search for online, hence our decision to include it in this online sources section of the guide.
We’ve established that grey literature is very tricky to define, so why enter its murky depths at all? You might actually have used grey literature without realising it, especially with a list of examples as long as your arm. It can be a positive addition to your research, but there are some pitfalls to be aware of too.
Grey literature is most definitely more integral to some disciplines than others. It tends to be more commonly included in research in applied subjects, where there is more likely to be a large amount of activity outside purely academic publishing. Take healthcare for example, where the cutting edge nature of the subject means that some material is out-of-date before it ever reaches a research journal. That isn’t to say that grey literature doesn’t apply to more theoretical subjects too, just that it tends to be less crucial to the research process in those disciplines. That nice broad definition means that we’ve probably all come across some form of grey literature in our time, but the discipline at hand would have dictated how much notice we would have taken.
As noted for healthcare, some disciplines thrive on cutting edge research and development. Including grey literature in your research can therefore ensure that you in turn stay up-to-date with the latest information and that there are no obvious gaps in your thinking.
A positive of grey literature is to counteract publication bias. Publication bias might refer either to papers being more likely to get published if they have exciting or game-changing results, or simply that particular academic publications prefer a certain type of submission (their ‘house style’, if you will). Most academic resources aren't deliberately trying to deceive (far from it), but there is also a certain amount of pressure to publish ‘worthy’ material.
What about all those research projects which don't go anywhere, which uncover ‘negative’ results or simply restate what we already know? Many of them might end up in the grey literature. Including grey literature in your search can therefore account for any potential bias in the more commercially published material, and ensure that you are also demonstrating discernment in your own research.
Be wary though: you need to be careful for quality control in grey literature. Precisely because these materials are not formally published and therefore subject to processes like peer review, there will be a huge variation in the quality within grey literature. You will therefore need to have your wits about you and make sure that you don't take anything on face value; question any bold claims and always compare the grey literature to its more formal relative to see if they make sense as a whole. There's a lot of value in using grey literature, but make sure it's never a substitute for the more academic literature.
Given its ephemeral nature, sadly there isn't a single place which indexes grey literature. Just as well really, as you might rightly accuse this guide of being unnecessary! There are however, lots of websites which index various forms of grey literature. Explore some of the options below.
You could also do worse than an effective Google search (or your alternative search engine of choice): a decent online search really can be a good approach for grey literature. You would definitely want to make use of the advanced search as that affords you a lot more control and complexity in your search. You could, for example, use Google initially to identify the websites of some relevant organisations or institutions, then use the advanced search to carry out a more specific search on those websites (with the ‘site or domain’ field).
Government reports or reports from academic organisations can often be found using Google Advanced Search. Try a domain filter for .edu or .ac.uk or limit to a specific file format; most reports are now published as PDFs, although you might find that some local government materials are made available as Word documents.
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