Grey literature is information that is produced by an organization where publishing is not its primary purpose and does not include journal articles. Including it in your work can boost its comprehensiveness and combat publication bias (the idea that research with negative or neutral findings are less likely to get published).
Examples of grey literature include:
Grey literature is not always indexed in traditional databases like PubMed or CINAHL, so it is important to know how to find it. Popular techniques include conducting structured Google searches, directly searching websites, using grey literature databases, and searching in Google Scholar.
Structured Google Searching is an effective technique for locating grey literature. Google gets a bad rap when it comes to searching: it's a search engine, not a database; its results are shaped by personalization, location, and opaque algorithms; and searches cannot be reliably replicated. These are valid and important criticisms, but with the right approach, it can be searched effectively using search commands, allowing you to target specific websites or types of websites, locate specific file types, or search in specific fields (e.g., in the title versus body).
Google's search commands are words, phrases, or symbols that help refine your search and attempt to bring order to the chaos that is the Google results page. They function similarly to Boolean operators, filters, and limiters in traditional databases. When used strategically, you can:
These commands can be typed directly into the regular Google search bar, and are especially useful when searching for grey literature, which is often hidden among ads or unhelpfully ranked results. They are also useful for searching the content of a specific website when its internal search function is limited or unreliable.
| Command | Function | Example |
|---|---|---|
| site: | restricts results to one website, allows for searching within a single website |
|
| intitle: | results will have the term(s) in the title of the webpage or document |
|
| intext: | results will have the term(s) in the body text of the page or document |
|
| filetype: | all results will have the specified file type |
|
| before: or after: | all results will be published either before or after the specified date |
|
| define: | the top of the search results will show a definition of the term |
|
| * (asterisk symbol) | acts like a wildcard or proximity search, not truncation. |
|
| ~ (tilde ) | includes synonyms of the term |
|
| - (minus) | excludes term, similar to NOT |
|
Website searching is a straightforward method of finding grey literature. Instead of battling Google's algorithms, ads, and ranking scheme, this method takes you straight to the source of grey literature using their search function or site navigation to find information.
This method has advantages:
But also has its challenges:
Tips for Effective Website Searching:
Another way to locate grey literature is through databases dedicated to it. These databases vary in scope: some are collections of all kinds of grey literature, while others focus on specific types like pre-print servers, clinical trial registries, repositories of dissertations, or government reports. Search functionality in these databases can vary. Some mirror traditional, academic databases with advanced searching, filters, and more; others use a custom Google search engine, providing middle-of-the-road search capabilities; and others offer a basic keyword search bar.
Using these databases can save time and boost relevance because their content is curated for research purposes. Additionally, their content is not always indexed by Google or Google Scholar, meaning that this may be the only way to find it. However, it's important to understand databases' structure and limitations, especially when it comes to how documents are in/excluded.
Google Scholar is one of the most popular places to search for scholarly information. It is a search engine, rather than a database, and indexes journal articles, grey literature, and more. Content is indexed using automated web crawlers that scan the internet for scholarly materials. For content to be indexed, it must be properly tagged and formatted such that it captures the attention of the crawlers. This means that some grey literature might not be indexed by Google Scholar and will not appear in a search.
Though Google Scholar is valuable for grey literature searching, it should not be your only source. Consider its strengths and limitations before deciding how to use it in your project:
Strengths
Limitations
Tips for Searching