Incorporating Information Status into Generation Ranking

Proceedings of the 47th Annual Meeting of the ACL and the 4th IJCNLP of the AFNLP, pages 817–825, Suntec, Singapore, 2-7 August 2009. c⃝2009 ACL and AFNLP Incorporating Information Status into Generation Ranking Aoife Cahill and Arndt Riester Institut f¨ur Maschinelle Sprachverarbeitung (IMS) University of Stuttgart 70174 Stuttgart, Germany {aoife.cahill,arndt.riester}@ims.uni-stuttgart.de Abstract We investigate the influence of informa- tion status (IS) on constituent order in Ger- man, and integrate our findings into a log- linear surface realisation ranking model. We show that the distribution of pairs of IS categories is strongly asymmetric. More- over, each category is correlated with mor- phosyntactic features, which can be au- tomatically detected. We build a log- linear model that incorporates these asym- metries for ranking German string reali- sations from input LFG F-structures. We show that it achieves a statistically signif- icantly higher BLEU score than the base- line system without these features. 1 Introduction There are many factors that influence word order, e.g. humanness, definiteness, linear order of gram- matical functions, givenness, focus, constituent weight. In some cases, it can be relatively straight- forward to automatically detect these features (i.e. in the case of definiteness, this is a syntactic prop- erty). The more complex the feature, the more dif- ficult it is to automatically detect. It is common knowledge that information status1 (henceforth, IS) has a strong influence on syntax and word or- der; for instance, in inversions, where the subject follows some preposed element, Birner (1994) re- ports that the preposed element must not be newer in the discourse than the subject. We would like to be able to use information related to IS in the automatic generation of German text. Ideally, we would automatically annotate text with IS labels and learn from this data. Unfortunately, however, to date, there has been little success in automati- cally annotating text with IS. 1We take information status to be a subarea of information structure; the one dealing with varieties of givenness but not with contrast and focus in the strictest sense. We believe, however, that despite this shortcom- ing, we can still take advantage of some of the in- sights gained from looking at the influence of IS on word order. Specifically, we look at the prob- lem from a more general perspective by comput- ing an asymmetry ratio for each pair of IS cate- gories. Results show that there are a large num- ber of pairs exhibiting clear ordering preferences when co-occurring in the same clause. The ques- tion then becomes, without being able to auto- matically detect these IS category pairs, can we, nevertheless, take advantage of these strong asym- metric patterns in generation. We investigate the (automatically detectable) morphosyntactic char- acteristics of each asymmetric IS pair and inte- grate these syntactic asymmetric properties into the generation process. The paper is structured as follows: Section 2 outlines the underlying realisation ranking system for our experiments. Section 3 introduces infor- mation status and Section 4 describes how we ex- tract and measure asymmetries in …

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