You've searched the databases! Congratulations! To achieve comprehensiveness, you've now got to consider strategies to supplement your database searching-- other ways of searching, and other places to search. 

Pearl-Growing (aka Snowballing or Reference Tracking)

• One of the best ways to search is to check the references of good articles
• Articles that are nodes of key research are called citation pearls, since they are a rich source of other citations
• You can also use a citation pearl as a source of keywords or relevant subject headings.

You can also search in the opposite direction, for later articles that have cited one of your citation pearls. Scopus and Web of Science are two databases that have this 'citing reference' feature. ​

Check out the video below for instructions on using Scopus to automatically locate & export references of your included articles: 

Handsearching

• The process of manually searching journal issues to check that nothing relevant has been left out of databased

Some journals, or specific article types (such as letters), are not indexed in databases.Handsearch a few key journals in your field to make sure your search is thorough.

The Cochrane Collaboration handsearches a number of major journals and conferences. Check their master list (link no longer live Jan 2019) to see if your journals are already being handsearched.

There are two good reasons to search other places (besides library databases) for evidence in a comprehensive search: 

1. Find references to key studies your database searching may have missed by reading (for example): 

• Newspapers
• Conference Proceedings
• Clinical Trial Records
• Theses and Dissertations
• Contacting authors directly 

2. Find studies, programs, or reports that aren't published in scientific journals or books by searching for (for example): 

• Government documents
• Charity or NGO white papers and other reports
• Pharmaceutical and other corporate reports
• Professional Association guidelines or reports

Many of these sources and documents are considered "grey literature". Whether you cite these sources as evidence, or they point you to evidence in the published literature is often based on contextual factors specific to your research. A librarian can help you figure it out. We've made things a bit easier by compiling key sources for grey literature, which you'll find on the left-hand side of this guide.