Categorizing web search results into comprehensible visual displays using meaningful and stable classifications can support user exploration, understanding, and discovery. We report on two formative studies in the domain of U.S. government web search that investigated how searchers use categorized overviews of search results for complex, exploratory search tasks. The first study compared two overview conditions vs. a control (N=18). The overviews were based on the federal government organizational hierarchy. With the overview conditions, participants noticed missing results more often than participants in the control. They also found pages of interest deeper within the results. The overview conditions received significantly higher subjective ratings. The second study compared an overview based on automated clustering vs. the government hierarchy overview (N=12), and the results suggest that domain knowledge and task influenced the preferred overview. The studies lend support to the use of compact overviews based on meaningful and stable categories tightly coupled with ranked result lists.
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