In distributed information retrieval, answers from separate collections are combined into a single result set. However, the collections may overlap. The fact that the collections are distributed means that it is not in general feasible to prune duplicate and near-duplicate documents at index time. In this paper we introduce and analyze the grainy hash vector, a compact document representation that can be used to efficiently prune duplicate and near-duplicate documents from result lists. We demonstrate that, for a modest bandwidth and computational cost, many near-duplicates can be accurately removed from result lists produced by a cooperative distributed information retrieval system.
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