Document retrieval can be considered as a basic but important tool for text mining that is capable of taking a user's information need into account. However, document retrieval is a hard task if multi-topic lengthy documents have to be retrieved with a very short description (a few keywords) of the information need. In this paper, we focus on this problem which is typical in real world applications. We experimentally validate that passage-based document retrieval is advantageous in such circumstances as compared to conventional document retrieval. Passage-based document retrieval is a kind of document retrieval which takes into account only small fractions (passages) of documents to judge the document relevance to the information need. As a passage-based method, we employ the method based on density distributions of keywords. This is compared with the following three conventional methods for document retrieval: the vector space model, pseudo-feedback, and latent semantic indexing. Experimental results show that the passage-based method is superior to the conventional methods if long documents have to be retrieved by short queries.
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