IV-5


a word and a multiplicity of concepts, since the former presumably

implies a unique meaning for that word while the latter implies a~rLbiguity.

        Even if almost all terms used in a given context are inherently

ambiguous, the juxtaposition of many multiple mappings can often identify

the appropriate concept classes with reasonable accuracy.  The relevant

categories will normally be reinforced, since they apply to many terms,

while the extraneous categories will be randomly distributed.

        Consider, for example, the set of terms: ~tbase't, ttbat", ~

tThit'1.   Each term is ambiguous, and a given multiple thesaurus mapping

may specify the correspondences shown in Table I.   In that table, three

categories are shown for the word "base1T, and two categories for each of

the other terms.    Despite the apparent ambiguities, a document identified

by the four original terms can nevertheless be assigned to the "baseball't

class with reasonable expectation of success, since the other categories

occur more or less at random for the given terms, whereas the "baseball"

class is always present.

        The principal advantages of synonym and phrase dictionaries for

purposes of content identification may then be summarized as follows:


        1)  they permit a consistent assignment of concept classes to items
            of information thereby replacing either keywords and index
            terms assigned to documents and search requests, or the words
            occurring in them;

        2)  they can often be used to resolve ambiguities by looking at
            the pattern of occurrence of the concepts;

        3)  they can serve for the analysis of many different subject
            fields and for different types of usage, since it is possible
            to adapt the dictionary to the particular search environment.