Utilizing external collections to improve retrieval performance is challenging research because various test collections are created for different purposes. Improving medical information retrieval has also gained much attention as various types of medical documents have become available to researchers ever since they started storing them in machine processable formats. In this paper, we propose an effective method of utilizing external collections based on the pseudo relevance feedback approach. Our method incorporates the structure of external collections in estimating individual components in the final feedback model. Extensive experiments on three medical collections (TREC CDS, CLEF eHealth, and OHSUMED) were performed, and the results were compared with a representative expansion approach utilizing the external collections to show the superiority of our method.

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Hierarchical text classification of a Web taxonomy is challenging because it is a very large-scale problem with hundreds of thousand categories and associated documents. Furthermore, the conceptual levels and training data availabilities of categories vary widely. The narrow-down approach is the state-of-the-art that utilizes a search engine for generating candidates from the taxonomy and builds a classifier for the final category selection. In this paper, we take the same approach but address the issue of using global information in a language modelling framework to improve effectiveness. We propose three methods of using non-local information for the task: a passive way of utilizing global information for smoothing, an aggressive way where a top-level classifier is built and integrated with a local model, and a method of using label terms associated with the path from a category to the root, which is based on our systematic observation that they are underrepresented in the documents. For evaluation, we constructed a document collection from Web pages in the Open Directory Project (ODP). A series of experiments and their results show the superiority of our methods and reveal the role of global information in hierarchical text classification.

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