Abstract
We discuss the complexity of explanation activity in human-human goal-directed dialogue, and suggest that this complexity ought to be taken account of in the design of explanation in human-computer interaction. We propose a general model of clarity in human-computer systems, of which explanation is one component. On the bases of: this model; of a model of human-intermediary interaction in the document retrieval situation as one of cooperative model-building for the purpose of developing an appropriate search formulation; and, on the results of empirical observation of human user-human intermediary interaction in information systems, we propose a model for explanation by the computer intermediary in information retrieval.