This paper focuses on the user and human‐computer interaction (HCI) aspects of the research based on the Okapi text retrieval system. Three experiments implementing different approaches to query expansion are described, highlighting the close relationship between the system’s functionality and different interface designs. The projects evaluated the retrieval effectiveness and usability of an automatic query expansion facility in a VT100 character‐based interface, and two different forms of interactive query expansion implemented in graphical user interface (GUI) environments with different windowing techniques. The experimental conditions, variables and constraints in undertaking operational user testing are discussed in relation to the interface features, and in terms of: the visibility of the system’s operations; the system/user control; and the cognitive load on the user. It is suggested that the quality and effectiveness of the search interaction for query expansion is dependent on resolving the tension between seemingly opposing interface and functional aspects, e.g. automatic vs interactive query expansion, explicit vs implicit use of a thesaurus, document vs query space.
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