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Clearly a vector space index can be used to answer Boolean queries, as long as the weight of a term in the document vector for is non-zero whenever occurs in . The reverse is not true, since a Boolean index does not by default maintain term weight information. There is no easy way of combining vector space and Boolean queries from a user's standpoint: vector space queries are fundamentally a form of evidence accumulation, where the presence of more query terms in a document adds to the score of a document. Boolean retrieval on the other hand, requires a user to specify a formula for selecting documents through the presence (or absence) of specific combinations of keywords, without inducing any relative ordering among them. Mathematically, it is in fact possible to invoke so-called -norms to combine Boolean and vector space queries, but we know of no system that makes use of this fact.
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