Failure analysis revealed the reasons why many records that were found by subject desciptors were not found by title keywords. First, although completely meaningless titles hardly ever appear, the title of a publication does not always offer sufficient clues for title keyword searching. In some cases, the subject of a publication can be more explicitely expressed by subject descriptors. A second and even more important task of subject descriptors is controlling the vocabulary. Many relevant titles cannot be retrieved by title keyword searching because of the wide diversity of ways to express a topic.
Descriptors take away the burden of vocabulary control from the user.