This dissertation investigates the use of categorized overviews of web search results, based on meaningful and stable categories, to support exploratory search. When searching in digital libraries and on the Web, users are challenged by the lack of effective overviews. Adding categorized overviews to search results can provide substantial benefits when searchers need to explore, understand, and assess their results. When information needs are evolving or imprecise, categorized overviews can stimulate relevant ideas, provoke illuminating questions, and guide searchers to useful information they might not otherwise find. When searchers need to gather information from multiple perspectives or sources, categorized overviews can make those aspects visible for interactive filtering and exploration. However, they add visual complexity to the interface and increase the number of tactical decisions to be made while examining search results.

Two formative studies (N=18 and N=12) investigated how searchers use categorized overviews in the domain of U.S. government web search. A third study (N=24) evaluated categorized overviews of general web search results based on thematic, geographic, and government categories. Participants conducted four exploratory searches during a two hour session to generate ideas for newspaper articles about specified topics. Results confirmed positive findings from the formative studies, showing that subjects explored deeper while feeling more organized and satisfied, but did not find objective differences in the outcomes of the search task. Results indicated that searchers use categorized overviews based on thematic, geographic, and organizational categories to guide the next steps in their searches.

This dissertation identifies lightweight search actions and tactics made possible by adding a categorized overview to a list of web search results. It describes a design space for categorized overviews of search results, and presents a novel application of the brushing and linking technique to enrich search result interfaces with lightweight interactions. It proposes a set of principles, refined by the studies, for the design of exploratory search interfaces, including "Organize overviews around meaningful categories," "Clarify and visualize category structure," and "Tightly couple category labels to search result list." These contributions will be useful to web search researchers and designers, information architects and web developers.