A data-driven methodology for motivating a set of coherence relations

The notion that a text is coherent in virtue of the `relations' that hold between its component spans currently forms the basis for an active research programme in discourse linguistics. Coherence relations feature prominently in many theories of discourse structure, and have recently been used with considerable success in text generation systems. However, while the concept of coherence relations is now common currency for discourse theorists, there remains much confusion about them, and no standard set of relations has yet emerged. The aim of this thesis is to contribute towards the development of a standard set of relations. We begin from an explicitly empirical conception of relations: they are taken to model a collection of psychological mechanisms operative during the tasks of reading and writing. This conception is eshed out with reference to psychological theories of skilled task performance, and to Rosch's notion of the basic level of categorisation. A methodology for investigating these mechanisms is then presented, which takes as its starting point a study of cue phrases|the sentence/clause connectives by which they are signalled. Although it is conventional to investigate psychological mechanisms by studying human behaviour, it is argued here that evidence for the constructs modelled by relations can be sought in an analysis of the linguistic resources available for marking them explicitly in text. The methodology is based on two simple linguistic tests: the test for cue phrases and the test for substitutability. Both tests are functional in inspiration: the former test identi es a heterogenous class of phrases used for linking one portion of text to another; and the latter test is used to discover when a writer is willing to substitute one of these phrases for another. The tests are designed to capture the judgements of ordinary readers and writers, rather than the theoretical intuitions of specialised discourse analysts. The test for cue phrases is used to analyse around 200 pages of naturally occurring text, from which a corpus of over 200 cue phrases is assembled. The substitutability test is then used to organise this corpus into a hierarchical taxonomy, representing the substitutability relationship between every pair of phrases. The taxonomy of cue phrases lends itself neatly to a model of relations as featurebased constructs. Many cue phrases can be interpreted as signalling just some features of relations, rather than whole relations. Small extracts from the taxonomy can be used systematically to determine the alternative values of single features; complex relation de nitions can then be formed by combining the values of many features. The thesis delivers results on two levels. Firstly, it sets out a methodology for motivating a set of relation de nitions, which rests on a systematic analysis of concrete linguistic data, and demands a minimum of theoretical assumptions. Also provided are the relation de nitions which result from applying the methodology. The new de nitions give an interesting picture of the variation that exists amongst cue phrases, and o er a number of innovative insights into text coherence. ii Acknowledgements I would like to take this opportunity to express my deepest thanks to my supervisors, Chris Mellish, Robert Dale and Steve Isard, for their immense help and encouragement over the last four years. Robert rst prompted me to think about the theoretical status of coherence relations; he led me to appreciate the need for a rm foundation for these constructs, as well as overseeing the assembly of the taxonomy of cue phrases. Chris was a huge help in guiding me through the theoretical interpretation of the taxonomy, in all its details. Steve provided a valuable perspective on the thesis as a whole. I would also like to thank my examiners, Donia Scott, Graeme Ritchie and Alex Lascarides, for their detailed comments and suggestions on the manuscript, and for a very interesting discussion during the viva. Many other people have been in uential in the development of the ideas presented here. A big thank you to all of the following, whose feedback and suggestions were much appreciated: Jean Carletta, Ed Hovy, Elisabeth Maier, Kathy McCoy, Johanna Moore, Megan Moser, Jon Oberlander, C ecile Paris, Orin Percus, Ted Sanders, Karen Sparck Jones, Wilbert Spooren, Manfred Stede, and Keith Vander Linden. Special thanks to Ted, for long discussions in several countries, over many glasses of Belgian beer. It seems to be easier to start a PhD than to nish one. This thesis would certainly not have been nished without the concerted e orts of a large and ruthless team of people, and I would like to say thank you to all of them for managing to force me into submission. Firstly to Chris, for the diverse rhetorical strategies he has employed to encourage me to nish, ranging from rational arguments and encouragements, through thinly veiled warnings, to direct threats and emotive appeals; and nally for eliciting from me a solemn oath to stop my research and start writing. Next, to the Engineering and Physical Sciences Research Council (EPSRC), for their generous support over three years; and also for their Faustian `four year deadline', which I have watched approaching with increasing horror but which has nally|I think|had its intended e ect on me. And lastly, to the assorted PhD students and denizens of the AI department, who have made my life on E oor most pleasant. Thanks are due to Alberto, Chris, Ed, Elena, Ian, Ion, Janet, Jean, Jeremy, Kim, Nam Seog, Ste an, and Wamberto, who have all been a great in uence on me during the past few years. Needless to say, any defects that remain in my character are solely my own responsibility. I would like to give special thanks to my family. Their constant support, particularly during the past year when I was running on empty, has been very important to me. Thank you Mum, Dad, Beck, Har, Jule. Thanks also to my wider family, in Britain, in New Zealand and in Tonga. Above all I would like to thank Mele, who has had to put up with a lot of ranting and raving on the subject of discourse structure just recently. Thank you for your love, and cheerfulness, and steadfastness. And especially for the notice pinned above my bed, reminding me of the important fact that No-One Reads PhD Theses. iii Declaration I hereby declare that I composed this thesis entirely myself and that it describes my own research. Alistair Knott Edinburgh December 3, 1996 iv