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Each database has its own set of subject headings, designed specifically for the literature from the field(s) of study the database contains. Knowing the difference between keywords and subject headings, as well as the advantages and disadvantages for both of them, can help you perform better searches.
natural language terms that describe your topic
able to be combined in any number of ways
lacking consistency in usage, definition, and sometimes spelling (e.g. GERD vs. GORD[U.K.])
either single words or phrases
used to search for matching words or phrases anywhere in the records the database contains (such as title, abstract, journal title)
used when no appropriate subject heading exists as an equivalent
sometimes either too broad or too narrow, resulting in either too many or too few results
reflective of recent phenomena in advance of when the subject headings are added
Examples of Keywords vs. Subject Headings
The chart below shows some examples of keywords and the equivalent subject headings in CINAHL and MEDLINE
Note that the Subject Headings in CINAHL and MEDLINE are not always the same. In the last example--LGBT--note that while CINAHL has a subject heading for the term, MEDLINE requires that two different subject headings have to be combined to create an equivalent.
Subject Headings are:
“controlled” vocabulary used by an organization (e.g. the National Library of Medicine) to describe the concepts in the literature collected by that organization or database (such as MEDLINE or CINAHL).
Consistent in their definition across the records in the database.
Less flexible and must be chosen from the thesaurus used by the database; if the incorrect subject heading is selected, none of the results will be relevant.
Only searched for in the subject heading field of the record.
Helpful for retrieving a set of articles with fewer irrelevant results
Slow to change--this means that the most recent changes in knowledge--on diseases, drugs, devices, procedures, concepts--may not be reflected in the controlled vocabulary.
Which Should I Use?
The simple answer to the question of whether you should use keywords or subject headings is: it depends. Some basic guidelines are:
If the term or topic is very recent, keywords may be the best option
If no Subject Heading exists for your term, or seems inadequate, use a keyword
If the keyword is too vague or broad, a Subject Heading may help focus your search and eliminate too many results
e.g. neuroses would be a very broad keyword search
If you want a very comprehensive literature search, you should use both a keyword and a subject heading