So now let's take a look at a contrasting situation. In the above example, if you were conducting a free-text keyword search in an image database on the word "spot", which of the above images would have come up?
If you answered all three you would be right! That's because all three contained the word "spot" somewhere within the database record. However, two of the items clearly have nothing to do with Jill's dog Spot.
Bibliographic databases work more or less the same way, in that a free-text keyword search will bring up all records that contain a given word anywhere within the database record. Sometimes the article will actually be "about" that topic, but other times - as in the 2 examples above - the article will have nothing to do with that topic, even though that word occurs within the database record.
Free-text keyword searches characteristically bring up larger numbers of results than subject heading searches. However, they also tend to bring up more articles that are "irrelevant" (i.e., off-topic) because they cannot differentiate between an incidental use of a word and when an article is actually about that topic.
Free-text keyword searches are great for the following types of topics:
If you are intending to conduct a thorough literature search, you would want to conduct BOTH a Subject Heading search AND a free-text keyword search. The Subject Heading search would bring up most of literature on your topic, but you may be able to tease a few more articles out of the database via the keyword search.
If a database does not employ a thesaurus, or if a given topic is not covered by a thesaurus term, then a keyword search is the only alternative.