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