Mapping your way through MEDLINE
Deidre Green
CMAJ 1999;160:1747
CMA members who search MEDLINE through OSLER (Ovid Search: Link to Electronic Resources) have become familiar with Ovid's unique vocabulary-mapping function. In other search interfaces mapping is absent, rudimentary or hidden from view, and the searcher will be unaware that a controlled vocabulary is being consulted to tailor results. An inadequate mapping system results in retrieval of "wrong hits," which discourages the user and wastes critical time. Fortunately, Ovid's mapping ability allows users to create precise search strategies.
OSLER automatically places users in Ovid's advanced mode, which provides several features essential to complex literature searching. In previous columns I have noted the importance of searching for individual concepts before combining the results to find references that contain all the concepts you seek. To activate mapping, terms must be searched one at a time. This allows the interface to exploit the large thesaurus of terms known as MeSH (medical subject heading) terms that was developed at the National Library of Medicine to index journals for MEDLINE and other databases. Mapping means that the Ovid software will race through the labyrinth of subject headings to zero in on the one (or several) that relate most completely to your request.
Every search starts at the Main Search Page. Let's suppose you are investigating the latest reports on carpal tunnel syndrome and other occupational injuries related to the daily use of computers in the workplace. Type repetitive strain injuries into the Keyword box and hit Perform Search. The screen that appears — see the accompanying illustration — indicates that the subject heading Cumulative Trauma Disorders should be used to retrieve information on your topic. Note that this term has already been ticked for you.
Because this subject heading may not sound quite like what you want, click on the term to see where it falls in the hierarchy of MeSH terms. You will then see a section of the MeSH Tree structure, and indexed under the MeSH term are 2 relevant narrower subjects, carpal tunnel syndrome and ulnar nerve compression syndrome. You can read the scope notes for each of these by clicking on the "i" beside each term. The definition provided for Cumulative Trauma Disorders verifies that it is indeed the appropriate choice.
Select Explode to broaden your search and to ensure that the system also searches for narrower terms, such as carpal tunnel syndrome. Hit Continue, and a list of subheadings that can be applied to your term will appear. Either select some subheadings, such as etiology or therapy, or simply hit Continue to gather all citations indexed under Cumulative Trauma Disorders, including all subheadings.
After Ovid has performed the search, you will be back on the Main Search screen, with the search strategy and number of references retrieved listed in the Search History Box. View the results by clicking on Display beside the number of references found. Next, perform a second search with the term computers. "Explode" the term Computers to ensure a broad retrieval of articles related to carpal tunnel syndrome. When the results appear on the Main screen, combine the searches with the Combine icon or simply type 1 and 2 into the Keyword Box; this will retrieve results from the two sets that contain both concepts searched.
Take another look at the illustration of the Mapping screen. Note that the words repetitive strain injuries also appear as a choice for selection. This is Ovid's method of allowing you to perform both Keyword, or text word, searching as well as MeSH searching. You can select both the MeSH term and the Keyword phrase that you entered, and combine them with the default 'OR'. That will not only retrieve references indexed with the MeSH heading but also references containing the Keywords within the title or abstract of the citation.
OSLER is a benefit exclusive to CMA members. Questions about registration or MEDLINE searching may be sent to cmalibrary@sympatico.ca. — Deidre Green, OSLER support librarian
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© 1999 Canadian Medical Association
