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In practice many of thesauri are constructed manually
...1 words which are deemed to be about the same topic are linked;2 words which are deemed to be about related things are linked
...The first kind of thesaurus connects words which are intersubstitutible,that is,it puts them into equivalence classes
...The second kind of thesaurus uses semantic links between words to,for example,relate them hierarchically
...However,methods have been proposed to construct thesauri automatically
...The basic relationship underlying the automatic construction of keyword classes is as follows:If keyword a and b are substitutible for one another in the sense that we are prepared to accept a document containing one in response to a request containing the other,this will be because they have the same meaning or refer to a common subject or topic
...It is not difficult to see that,based on this principle,a classification of keywords can be automatically constructed,of which the classes are used analogously to those of the manual thesaurus mentioned before
...1 replace each keyword in a document and query representative by the name of the class in which it occurs;2 replace each keyword by all the keywords occurring in theclass to which it belongs
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searching
...One last distinction,the vocabulary of an index language may be controlled or uncontrolled
...The index language which comes out of the conflation algorithm in the previous section may be described as uncontrolled,post coordinate and derived
...There is much controversy about the kind of index language which is best for document retrieval
...Probably the most substantial evidence for automatic indexing has come out of the SMART Project 1966
...The document representatives used by the SMART project are more sophisticated than just the lists of stems extracted by conflation
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