Snowballing, also reference harvesting or pearl growing, is a term used by expert and systematic searchers to refer to locating, tracking and chasing down references in footnotes and bibliographies. 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 bibliography sources, however, snowballing is only one technique used by health librarians in their information retrieval activities.
Snowballing techniques can be used in conjunction with hand-searching and systematic review searching techniques to ensure high levels of recall for the literature review. 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 to what you have 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.