A pearl is 'an authoritative article, typically identified by experts, of particular relevance to the topic of inquiry that can be used to search for relevant and authoritative materials sharing common characteristics with the original pearl'.¹

Pearl growing (also known as snowballing) is a literature-searching strategy that builds on information that you already have, ie a pearl. In particular, if you already have a paper that is relevant to your work (your "starting paper"), you can use clues provided in the paper and in its database record to find more like it.

Especially useful pearl growing clues include:

• Subject headings - The database record for your starting paper will often include subject headings (eg MeSH terms); searching using these subject headings should help find other papers on the same topic as your initial paper.
• Keywords - Words used in the text of the paper, or provided as a separate list, may be useful search terms to find similar papers.
• Author names - A researcher who has written a paper on a topic will likely have written other papers on the same or similar topics, so looking up the other work of the authors of your starting paper may lead you to other useful papers.
• References - The list of references at the end of your starting paper will likely include other papers on the same topic. Note that these papers will be older than your starting paper so looking at them involves moving backward through the literature.
• Citations - Many databases will provide lists of papers that cite your starting paper and include it in their list of references: Like papers in the reference list, many will be on the same topic, but note that citing papers will be more recent than your starting paper so looking at them involves moving forward through the literature.

General steps in using the pearl growing strategy

1. Find the database record for your starting paper, eg by searching for its title.
2. Identify useful clues in the record - the MeSH terms, authors' names, cited by etc.
3. Search the database using those clues.
4. Examine the records retrieved in your searches, and identify articles of interest.
5. If desired, use the records of newly-found articles as starting points for new searches.
6. Repeat until you have enough articles, or you no longer seem to be finding anything new. You know you have done a good search when further searching keeps leading you back to a few key papers on the topic.

Pearl growing in various databases

Databases vary in how they facilitate your use of features like subject headings, citing articles, or author names to find papers similar to one you already have. They may also include tools to automate searching, such as:

• Hyperlinked author names that automatically find all papers by a particular author.
• A Cited by link that automatically looks for papers in the database that have cited your starting paper.
• A Related items link that automatically looks for papers with similar keywords or other features to your starting paper.

There are two things to be wary of about these features:

• Hyperlinks and buttons can only recover records within the particular database: Other databases may include other relevant papers, and must be searched separately. (It is therefore good practice to search several databases when pearl growing.)
• Different databases may execute similar functions (eg finding related/similar papers) in completely different ways. Differences may include where and what buttons or links you as the searcher must use, as well as behind-the-scenes differences in how the automated functions go about identifying "similar" papers.

Let's see how this works by starting with a paper...

1. Booth A. Searching for qualitative research for inclusion in systematic reviews: a structured methodological review. Syst Rev. 2016;5(1):74.