Experiments on Lexical Chaining for German Corpora: Annotation, Extraction, and Application

Converting linear text documents into documents publishable in a hypertext environment is a complex task requiring methods for segmentation, reorganization, and linking. The HyTex project, funded by the German Research Foundation (DFG), aims at the development of conversion strategies based on text-grammatical features. One focus of our work is on topic-based linking strategies using lexical chains, which can be regarded as partial text representations and form the basis of calculating topic views, an example of which is shown in Figure 1. This paper discusses the development of our lexical chainer, called GLexi, as well as several experiments on two aspects: Firstly, the manual annotation of lexical chains in German corpora of specialized text; secondly, the construction of topic views. The principle of lexical chaining is based on the concept of lexical cohesion as described by Halliday and Hasan (1976). Morris and Hirst (1991) as well as Hirst and St-Onge (1998) developed a method of automatically calculating lexical chains by drawing on a thesaurus or word net. This method employs information on semantic relations between pairs of words as a connector, i.e. classical lexical semantic relations such as synonymy and hypernymy as well as complex combinations of these. Typically, the relations are calculated using a lexical semantic resource such as Princeton WordNet (e.g. Hirst and St-Onge (1998)), Roget’s thesaurus (e.g. Morris and Hirst (1991)) or GermaNet (e.g. Mehler (2005) as well as Gurevych and Nahnsen (2005)). Hitherto, lexical chains have been successfully employed for various NLP-applications, such as text summarization (e.g. Barzilay and Elhadad (1997)), malapropism recognition (e.g. Hirst and St-Onge (1998)), automatic hyperlink generation (e.g. Green (1999)), question answering (e.g. Novischi and Moldovan (2006)), topic detection/topic tracking (e.g. Carthy (2004)). In order to formally evaluate the performance of a lexical chaining system in terms of precision and recall, a (preferably standardized and freely available) test set would be required. To our knowledge such a resource does not yet exist–neither for English nor for German. Therefore, we conducted several annotation experiments, which we intended to use for the evaluation of GLexi. These experiments are summarized in Section 2 . The findings derived from our annotation experiments also led us to developing the highly modularized system architecture, shown in Figure 4, which provides interfaces in order to be able to integrate different pre-processing steps, semantic relatedness measures, resources and modules for the display of results. A survey of the architecture and the