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By Leah Anderson, MLS
Editor’s Note: This is the second in a series of articles on using PubMed, the National Library’s search service, which includes Medline. It is free via the Internet and can be found at www.nebi.nlm.nih.gov/PubMed.
All literature database strategies involve a balance between the number of references retrieved and the relevancy of the results to the question at hand. For many librarians, a good search is one in which the vast majority of the references found address the requester’s question. We are more confident that all references on that topic were retrieved because the search was performed broadly enough to pick up a bit off the topic. If the search yielded only references directly on the topic, then one worries that something was missed. The following are tips for searching the PubMed database that can improve relevancy over retrieval.
Never search for your terms using the parameter "All Fields." Unfortunately, "All Fields" is the default setting in PubMed. You have to manually change it to something else but your searches will be more focused. There are many fields in a PubMed record that have nothing to do with the topical content of the article. Thus, using "All Fields" increases retrieval but decreases relevancy. A good example: a researcher wanted all references to the molecule, JAK. He searched for JAK using "All Fields" and couldn’t understand why he got more than 1000 references when only about a third of them discussed his molecule. It turns out that the initials, JAK, are also the three-letter code (in its own field) for the Journal of Neural Transmission Supplement. By using "All Fields," he retrieved articles discussing his molecule as well as every paper published in that journal.
For broadly searching by subject, use the "Text Word" field. When using this field, your terms will be searched only in the article’s title, abstract, or medical subject headings (MeSH). The researcher above should have used this field. Although better than "All Fields," this approach is still likely to give you more retrieval than relevancy. The more general the search terms, the less targeted your results will be. For example, searching "ambulatory patient groups" as a text word will give you a lot of references having nothing to do with that subject.
To gain more relevancy, search your terms in the field "Title Word." This quick approach is especially productive when a few articles focusing on a topic are needed and comprehensiveness is not important. If an article has your terms in its title, you are reasonably assured that it addresses your topic.
Searching in the MeSH field is an excellent way to maximize relevancy and retrieval. PubMed uses the MeSH Browser to select the appropriate MeSH terms.
To quickly narrow a search, eliminate all foreign language articles. Select the "Language" field and enter "English." This can reduce results by as much as a third.
Narrow the search by date. The PubMed database goes back to 1966. This can greatly increase retrieval for many topics. If you are searching on a new technique or molecule, this isn’t much of an issue. The easiest way to narrow by date is to use the "Entrez Date Limit" area of the current query section of the search screen. The default is "no limit." Clicking on the box reveals your choices ranging from the last 30 days to 10 years. This method can be misleading. It means you are restricting your results to the date the references were entered into the database—not the date published. Thus, selecting the last 30 days as your limit means those references retrieved were entered into PubMed within the last 30 days regardless of when the articles were published. On a related note, this is an easy way to check PubMed routinely for new references on your topic. To limit your results to a particular publication year only, select the field "Publication Date" and enter the year.
You can also limit to review articles. Select the field "Publication Type" and enter "review." Be careful not to enter the plural form "reviews."
Many clinicians are not interested in animal research, so limiting topics to human research is another trick. Select the field "MeSH Term" and enter "human." Conversely, if you are specifically looking for animal research, limit to "animal." Many articles are given both human and animal MeSH terms since both are addressed in the article. In a similar vein, limit by male or female if it is relevant to your topic. (Ms. Anderson is Medical Librarian for the Health Sciences Library, Sequoia Health Services, Redwood City, CA.)