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)

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.

• One of the best ways to search is to check the references of good articles. This is referred to as backward reference tracking/backward citation searching. 
• You can also search in the opposite direction, for later articles that have cited one of your citation pearls. This is referred to as forward reference tracking/forward citation searching. 

Scopus and Web of Science are two databases that have this 'citing reference' feature. Citation Chaser is a tool which provides similar features, finding the articles that cite and are cited by your articles (i.e., citation pearls). Scopus and Citation Chaser, two options for reference tracking, are discussed below. 

1. Scopus

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

2. Citation Chaser

Citation Chaser is an automated tool that retrieves articles that cite and are cited by your articles (i.e., citation pearls). As a single-purpose tool, Citation Chaser limits itself to reference tracking, making it easy to use. Citation Chaser searches the Lens.org database, which includes PubMed, PubMed Central, CrossRef, Microsoft Academic Graph, and CORE, to find articles. With Citation Chaser:

• Multiple articles or 'citation pearls' can be uploaded in bulk using a CSV/RIS file. 
• Retrieved articles can be analyzed and visualized as citation networks. 

The following steps outline how to use Citation Chaser: 

1. Input the unique article identifiers (e.g., DOIs, PMIDs, PMCIDs) associated with your articles into the "Article input" tab. There are several options for entering articles, e.g., uploading a CSV file of DOIs.
2. Select the "References" tab to view articles cited by your chosen articles. 
3. Select the "Citations" tab to view articles that cite your chosen articles. 
4. Export the results as RIS files.

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.