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Reports on two studies in the second Text Retrieval Conference (TREC-2) program that investigated the effect on retrieval performance of combination of multiple representations of TREC topics. Six methods of query combinations and the combination of the same queries on different databases are examined. Results show that combining evidence from different systems leads to performance improvement. (Author/AEF)

This paper discusses the complexity of explanation activity in human-human goal-directed dialogue and suggests that this complexity ought to be taken into account in the design of explanation in human-computer interaction. A general model of clarity in human-computer systems is proposed, incorporating explanation as one component. On the basis of this model, a model of human-intermediary interaction in the document retrieval situation which emphasizes cooperative model-building for the purpose of developing an appropriate search formulation, and the results of empirical observation of human user/human intermediary interaction in information systems, a model is proposed for explanation by the computer intermediary in information retrieval. (22 references) (Author/GL)