Using machine learning to improve information access
The explosion of on-line information has given rise to many query-based search engines (such as Alta Vista) and manually constructed topic hierarchies (such as Yahoo!). But with the current growth rate in the amount of information, query results grow incomprehensibly large and manual classification in topic hierarchies creates an immense information bottleneck. Therefore, these tools are rapidly becoming inadequate for addressing users' information needs. 
In this dissertation, we address these problems with a system for topical information space navigation that combines the query-based and taxonomic approaches. Our system, named SONIA (Service for Organizing Networked Information Autonomously), is implemented as part of the Stanford Digital Libraries testbed. It enables the creation of dynamic hierarchical document categorizations based on the full-text of articles. Using probability theory as a formal foundation, we develop several Machine Learning methods to allow document collections to be automatically organized at a topical level. First, to generate such topical hierarchies, we employ a novel probabilistic clustering scheme that outperforms traditional methods used in both Information Retrieval and Probabilistic Reasoning. Furthermore, we develop methods for classifying new articles into such automatically generated, or existing manually generated, hierarchies. In contrast to standard classification approaches which do not make use of the taxonomic relations in a topic hierarchy, our method explicitly uses the existing hierarchical relationships between topics, leading to improvements in classification accuracy. Much of this improvement is derived from the fact that the classification decisions in such a hierarchy can be made by considering only the presence (or absence) of a small number of features (words) in each document. The choice of relevant words is made using a novel information theoretic algorithm for feature selection. Many of the components developed as part of SONIA are also general enough that they have been successfully applied to data mining problems in different domains than text. 
The integration of hierarchical clustering and classification will allow large amounts of information to be organized and presented to users in a individualized and comprehensible way. By alleviating the information bottleneck, we hope to help users with the problems of information access on the Internet.