IV-21


      a)  The Phrase Dictionaries

      Both the regular as well as the null thesauruses are based on entries

corresponding either to sinnle words or to single word stems.   In attempting

to perform a subject analysis of written text, it is possible however, to

go frrther by trying to locate "phrases" consisting of sets of words which

are judged to be important in a given subject area. For example, in the

field of computer science, the concepts of "program" and ~ may

mean many things to many people.  On the other hand, the phrase concept

which results from a combination of these individual words, that is,

1'progremming language" has a much more specific connotation.  Such phrases

can be used for subject identification by building phrase dictionaries to

be used in locating combinations of concepts, rather than individual concepts

alone.   Such phrase dictionaries would then normally include pairs, or triples,

or quadruples of words or concepts, corresponding in written texts to the

more likely noun and prepositional phrases which may be expected to be

indicative of subject content in a given topic area.

      Ma;ny different strategies can be used in the construction of phrase

dictionaries.  For example, it is possible to base phrase dictionaries on

c~~ibinations of high-frequency words or word stems occurring in documents and

search requests; alternatively, one may want to use a thesaurus before appeal

is made to a phrase dictionary.  U~ider those circumstances, the phrase

dictionary would then be based on con~inations of concept categories included

in the thesaurus, rather than on combinations of words.

      Furthermore, given the availability of a phrase dictionary one can

recognize the presence of phrases in a given text under a variety of cir-

cumstances: for example, the existence of a phrase may be recognized

whenever the phrase components are present within a given document, regard-