# Understanding Query Combination Behavior in Exploratory Searches
## 3 A Hierarchy of Query Combination Behavior
### A Hierarchy of QCB  
To explore the space of QCB types, in this section we develop a hierarchy and categorization scheme for this behavior. For this purpose, we first brainstormed and listed candidates of QCB type as extensively as possible with our experiences. The list served as a prototype of our hierarchy. We then gradually revised the prototype. Five exploratory search researchers took part in this task. Researchers were asked to collaboratively judge 100 QCB sessions by examining the queries, the search results pages, and the clicked-on web pages. These QCB sessions were identified and randomly picked from either AOL search logs [30] or Sogou search logs [31]. Categories were revised, or new categories were added, if a QCB session fell out of the prototype hierarchy. For example, the previous QCB type \"accurate description\" was removed as unrepresentative (almost all QCB will fall into this category, providing evidence that this is a common feature of QCB). If categories were modified or new categories were added, another 100 QCB sessions were presented for judging. The judge-revise-judge cycle was repeated until all 100 randomly picked QCB sessions were classified into our hierarchy. As the prototype hierarchy of QCB types was gradually modified, some QCB types were split into sub-types, and finally a two-layer hierarchy of QCB naturally emerged. Figure 2 shows the hierarchy. The first layer in Figure 2 is the category layer, which contains three QCB categories: the topic specialization category, the relation discovery category, and the query diversification category. The second layer is the pattern layer. For example, the topic specialization category can be divided into topic specialization query first pattern and dominative query first pattern. To better understand the space of QCB types, we illustrate the hierarchy in the remainder of this section. Note that, to better illustrate the examples of QCB, we omit queries other than CPQ and CBQ in the following examples. 1. Topic specialization The topic specialization category of QCB demonstrates a requirement by the user to limit the search results for CPQ into specific topics, represented by other CPQs. CPQs in this category can be divided into two types: Dominative queries are a particular type of CPQ, the search results for which cover the objects that users are seeking. Topic specialization queries are also a particular type of CPQ, the search results for which correspond to the topic that the user has in mind. To be considered as being under the topic specialization category of QCB, some CPQs must correspond to the search object that the user already has in mind, while the others correspond to the topic of the objects which he or she wants. We found that search results for the dominative query are usually across different topics. Users exhibited this category of QCB after they learned that their information needs could be met by synthesizing topics and subjects. Two patterns naturally emerged according to whether the time sequence, dominative query, or topic specialization query came first. We found that these patterns correspond to two types of search goals: exploring the topics of their search objects or exploring the search objects in their topics. We compared the patterns in this QCB category as follows: * Dominative query first: This pattern describes users' issue dominative queries in advance of the topic specialization queries. Users who exhibited this pattern of QCB were exploring the topics of their information needs. For example, the QCB session such as \(<\)\"online games\", \"pinball\", \"pinball online games\"\(>\), where the user was exploring to find online games, is judged as a dominative query first pattern. In this session, the CPQ \"online games\" contains the subject that the user wants in mind, which is considered as a dominative query, and another CPQ, \"pinball\", contains the topic information, which expresses that the type of online game he or she searched for was \"pinball\".