COMMUNICATIONSOF THE ACMApril 2006/Vol. 49, No. 4 41EXPLORATORY SEARCH: FROM FINDING TOUNDERSTANDINGrom the earliest days of computers, search has been afundamental application that has driven research anddevelopment. For example, a paper published in theinaugural year of the IBM Journal 36 years ago out-lined challenges of text retrieval that continue to thepresent [4]. Today’s data storage and retrievalapplications range from database systems thatmanage the bulk of the world’s structured datato Web search engines that provide access topetabytes of text and multimedia data. Ascomputers have become consumer products and theInternet has become a mass medium, searching theWeb has become a daily activity for everyone fromchildren to research scientists. By Gary MarchioniniFResearch tools critical for exploratory search success involve the creation of new interfaces that move the process beyond predictable fact retrieval.42 April 2006/Vol. 49, No. 4 COMMUNICATIONSOF THE ACMAs people demand more of Web services, shortqueries typed into search boxes are not robust enoughto meet all of their demands. In studies of early hyper-text systems, we distinguished analytical search strate-gies that depend on a carefully planned series ofqueries posed with precise syntax from browsingstrategies that depend on on-the-fly selections [7].The Web has legitimized browsing strategies thatdepend on selection, navigation, and trial-and-errortactics, which in turn facilitate increasing expectationsto use the Web as a source for learning andexploratory discovery. This overall trend toward moreactive engagement in the search process leads theresearch and develop-ment community tocombine work inhuman-computer inter-action (HCI) and infor-mation retrieval (IR).This article distinguishesexploratory search thatblends querying andbrowsing strategies fromretrieval that is bestserved by analyticalstrategies, and illustratesinteractive IR practicesand trends with examples from two user interfacesthat support the full range of strategies. Exploratory search. Search is a fundamental lifeactivity. All organisms seek sustenance and propaga-tion and Maslow’s classic hierarchy of needs theorypredicts that once people fulfill basic physiologicalneeds, we seek to fulfill social and psychological needsto belong and to know our world. These higher-levelneeds are often informational and this in turnexplains why information resources and communica-tion facilities are so sophisticated in developed soci-eties. Ahierarchy of information needs mayalso be defined that ranges from basic facts that guideshort-term actions (for example, the predicted chancefor rain today to decide whether to bring an umbrella)to networks of related concepts that help us under-stand phenomena or execute complex activities (forexample, the relationships between bond prices andstock prices to manage a retirement portfolio) to com-plex networks of tacit and explicit knowledge thataccretes as expertise over a lifetime (for example, themost promising paths of investigation for the sea-soned scholar or designer). For these respective layersof information needs, we can define kinds of infor-mation-seeking activities, each with associated strate-gies and tactics that might be supported withcomputational tools. Figure 1 depicts three kinds of search activities thatwe label lookup, learn, and investigate; and highlightsexploratory search as especially pertinent to the learnand investigate activities.1These activities are repre-sented as overlapping clouds because people mayengage in multiple kinds of search in parallel, andsome activities may be embedded in others; for exam-ple, lookup activities areoften embedded in learnor investigate activities.The searcher views theseactivities as tasks, so weuse “task” in the followingdiscussion. Lookup is the mostbasic kind of search taskand has been the focus ofdevelopment for databasemanagement systems andmuch of what Web searchengines support. Lookuptasks return discrete andwell-structured objectssuch as numbers, names, short statements, or specificfiles of text or other media. Database managementsystems support fast and accurate data lookups inbusiness and industry; in journalism, lookups arerelated to questions of who, when, and where asopposed to what, how, and why questions. Inlibraries, lookups have been called “known item”searches to distinguish them from subject or topicalsearches. Most people think of lookup searches as “factretrieval” or “question answering.” In general, lookuptasks are suited to analytical search strategies thatbegin with carefully specified queries and yield preciseresults with minimal need for result set examinationand item comparison. Clearly, lookup tasks have beenamong the most successful applications of computersand remain an active area of research and develop-ment. However, as the Web has become the informa-tion resource of first choice for information seekers,people expect it to serve other kinds of informationneeds and search engines must strive to provide ser-vices beyond lookup.March fig 1 (4/06)- 26.5 picasFigure 1. Search Activities.March fig 1 (4/06) - 19.5 picasInvestigateLearnLookupExploratory SearchFact retrievalKnown item searchNavigationTransactionVerificationQuestion answeringKnowledge acquisitionComprehension/InterpretationComparisonAggregation/IntegrationSocializeAccretionAnalysisExclusion/NegationSynthesisEvaluationDiscoveryPlanning/ForecastingTransformationInvestigateLearnLookupExploratory SearchFact retrievalKnown item searchNavigationTransactionVerificationQuestion answeringKnowledge acquisitionComprehension/InterpretationComparisonAggregation/IntegrationSocializeAccretionAnalysisExclusion/NegationSynthesisEvaluationDiscoveryPlanning/ForecastingTransformationFigure 1. Search activities.1There are many important theoretical models of information search, for example,Saracevic summarizes Belkin’s and Ingrewsen’s in his stratified model [9].COMMUNICATIONSOF THE ACMApril 2006/Vol. 49, No. 4 43Searching to learn is increasingly viable as more pri-mary materials go online. Learning searches involvemultiple iterations and return sets of objects thatrequire