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Snowballing, reference harvesting or pearl growing, is a term used by expert searchers to refer to locating, tracking and chasing down references in the footnotes and bibliographies of articles and other documents. It is a continuous, recursive process of gathering, searching, scanning and aggregating references. As a term used to refer to the retrieval of citations in references and bibliographic sources, snowballing is only one technique used by health librarians in their information retrieval activities. In the harvesting or citation gathering phase of a search, documents that match a topic or information need are used to scan some of the most-commonly cited research. From these documents, searchers find other keywords, descriptors and themes to use in their subsequent "gold standard" searches.
Synonyms for snowballing
Snowballing is sometimes referred to as reference harvesting or pearl growing search method; here are some other terms that evoke similar kinds of search strategies:
Snowballing techniques can be used in conjunction with hand-searching and systematic review searching techniques to ensure high levels of recall for literature reviews. The best starting point for snowballing is a group of relevant documents (say, 4 or 5) because these documents are more likely to cite the kind of research and lead you to documents that are similar in nature to those you've already located. Authors, titles, abstracts and captions within the document, with descriptors and other metadata can also be examined, and those terms can then be used to reformulate search terms in further searches. It is also a good idea to study the reference lists of any new documents that have cited references, and their lists, and so on. Dissertations and theses are rich sources for locating additional references, grey studies, grey data and grey literature.
Persistence is absolutely essential in using the snowballing technique to effectively locate materials for the systematic review. Information retrievalists and health librarians know instinctively that relevant documents may not always be found at one search sitting. Initial searches on the web, for example, may only provide clues that will help to inform where a searcher might look for relevant documents and grey literature over the course of several search sessions. Fugitive or literature in the deep web are hard to find because they are not indexed in the major databases; perhaps it can be said that snowballing is, in part, a way to find these materials or as part of larger retrieval strategies.
Information retrievalists and health librarians are advised to document their searches in the form of checklists, spreadsheets or other formats to ensure that their searches are reproducible. This will not only help you to trace and document your search results, your explicit description of your harvesting activities will form part of your reporting.
Digital snowballing is a way to extend the search for articles. In databases such as PubMed, use the Related Citations feature for articles that are similar to the original article you are reviewing. In databases such as Web of Science and Google scholar, examine the articles that cite your original research, or the authors of the paper, as these articles may be related. These are just some of the techniques used by health librarians to find items that might be missed by conventional online search methods. With snowballing, think of rolling a snowball metaphorically and starting small in your information retrieval activities, and slowly but surely finding and picking up absolutely everything in your path.