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G high increase line a document containing such a term

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Unformatted text preview: levant than a document with 1 occurrence of the term.   But not 10 *mes more relevant.   Relevance does not increase propor*onally with term frequency. NB: frequency = count in IR Introduc)on to Informa)on Retrieval Sec. 6.2.1 Document frequency   Rare terms are more informa*ve than frequent terms   Recall stop words   Consider a term in the query that is rare in the collec*on (e.g., arachnocentric)   A document containing this term is very likely to be relevant to the query arachnocentric   → We want a high weight for rare terms like arachnocentric.   The score is 0 if none of the query terms is present in the document. 3 Introduc)on to Informa)on Retrieval Sec. 6.2.1 Introduc)on to Informa)on Retrieval S ec. 6.2.1 Document frequency, con*nued idf weight   Frequent terms are less informa*ve than rare terms   Consider a query term that is frequent in the collec*on (e.g., high, increase, line)   A document containing such a term is more likely to be relevant than a document that doesn t   But it s not a sure indicator of relevance.   → For frequent terms, we want high posi*ve weights for words like high, increase, and line   But lower weights than for rare terms.   We will use document frequency (df) to capture this.   dft is the document frequency of t: the number of documents that contain t Introduc)on to Informa)on Retrieval S ec. 6.2.1 idf example, suppose N = 1 million term dft 1 animal 1,000 fly Introduc)on to Informa)on Retrieval Effect of idf on ranking...
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