Resolving Direct and Indirect Anaphora for Japanese Definite
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Resolving Direct and Indirect Anaphora for Japanese Definite Noun Phrases Naoya Inoue Ryu Iida Kentaro Inui Yuji Matsumoto NAIST, Japan Tokyo Institute of Technology, Japan NAIST, Japan NAIST, Japan [email protected] [email protected] [email protected] [email protected] Abstract An anaphoric relation can be either direct in parallel and classify the anaphora type. How- or indirect. In some cases, the antecedent ever, these issues have received little attention in being referred to lies outside the discourse the literature. to which its anaphor belongs. Therefore, In this paper, we regard the task of nominal an anaphora resolution model needs to con- anaphora resolution as a combination of two dis- sider the following two decisions in paral- tinct but arguably interdependent subtasks: lel: (i) antecedent selection–selecting the • Antecedent selection: the task of identifying the antecedent itself, and (ii) anaphora type antecedent of a given input definite NP, and classification–classifying exophora, direct • Anaphora type classification: the task of judg- anaphora, or indirect anaphora. However, ing whether the anaphor is direct anaphoric, in- the anaphora type classification has re- direct anaphoric or exophoric. ceived little attention in the literature. In Given this task decomposition, the following unex- this paper, we address this issue taking plored issues immediately arise: Japanese as our target language. Our find- • Whether the model for antecedent selection ings were: first, an antecedent selection should be designed and trained separately for model should be trained separately for each direct anaphora and indirect anaphora or if it anaphora type. Second, the best candidate can be trained as a single common model, antecedent selected by an antecedent selec- • How the two subtasks can be best combined tion model provides contextual information (e.g. which subtask should be carried out first), useful for anaphora type classification. In and consequence, antecedent selection should • Which context information is useful for be carried out before anaphora type clas- anaphora type classification. sification. In this paper, we explore these issues taking 1 Introduction Japanese as our target language. Anaphora resolution has been studied inten- Definite NPs in Japanese behave similarly to sively in recent years because of its significance those in English; that is, a definite NP may bear in many NLP applications such as information a direct anaphoric relation but may also bear an in- extraction and machine translation. In nominal direct anaphoric relation to its antecedent as shown anaphora resolution, an anaphor (typically a defi- in examples (4) and (5). nite noun phrase) and its antecedent hold either a (4) 新型車 が発売された。このミニバン は燃費が非常に direct anaphoric relation (e.g., coreference) or an よい。 A new car was released. The minivan has good gas indirect relation (e.g., bridging reference (Clark, mileage. 1977)). In example (1), the anaphor the CD refers (5) 家具屋で 机 を見た。そのデザイン は素晴らしかった。 to Her new album directly. I saw a desk in a furniture shop. The design was mar- (1) Her new album was released yesterday. velous. I want to get the CD as soon as possible. In contrast, the CD refers to her new song indi- “このミニバン (the minivan)” refers to “新型車 rectly in example (2). (a new car)” directly in (4), while “そのデザイン 机 (2) The artist announced her new song. (the design)” refers to “ (a desk)” indirectly in I want to get the CD as soon as possible. (5). In Japanese, however, the problem can be even In cases such as example (3), an antecedent re- more complex because a definite NP is not always ferred to is not in the same discourse. marked by a definiteness modifier. In an example (3) I want to get the CD as soon as possible. (6), a bare NP 大統領 (literally, President) refers to 韓国大統領 We call the reference in example (1) direct (Korean President). anaphora; in example (2) indirect anaphora; and (6) 今月 4 日、韓国大統領 が来日した。大統領 は翌日の 記者会見で新プランの詳細を語った。 in example (3) exophora. In anaphora resolution, Korean President visited Japan on the 4th this month. besides identifying the antecedent itself, it is there- (The) President talked about the details of his new plan fore necessary to consider these three possibilities at the news conference next day. Anaphora Type Classifier Antecedent Selection Model Anaphor Anaphor Anaphor Anaphor Anaphor Candidates Information Anaphor Direct Indirect Direct Indirect Anaphora Anaphora Anaphora Anaphora Single Direct Indirect Direct Indirect Anaphora Anaphora Anaphora Anaphora Direct Indirect Direct Indirect Direct Indirect Direct Indirect Anaphora Anaphora Anaphora Anaphora Anaphora Anaphora Direct Indirect Direct Indirect Anaphora Anaphora Anaphora Anaphora Anaphora Anaphora Indirect Direct Exophora Exophora Anaphora Anaphora Exophora Exophora Exophora Exophora a-Classify-then-Select c-Classify-then-Select s-Select-then-Classify d-Select-then-Classify i-Select-then-Classify p-Select-then-Classify (aC/S) (cC/S) (sS/C) (dS/C) (iS/C) (pS/C) Figure 1: Anaphora resolution models Furthermore, it is sometimes difficult even for hu- and the antecedent to resolve direct anaphora. In man annotators to determine the definiteness of a example (1), the resolution model has to know bare NP. Therefore, our study for now deals with that CD and album are synonymous. For indirect the resolution of noun phrases modified by a defi- anaphora, on the other hand, the model is required niteness modifier, i.e., この (this) + NP, その (the) to recognize such semantic relations as part-whole + NP and あの (that) + NP. This problem alone is and attribute-of as in example (2), where knowing large enough, because noun phrases modified by a that CD is an object related to song is crucial. This definiteness modifier account for a large proportion expectation is supported empirically by our exper- of occurrences of nominal anaphora in Japanese iments as reported in section 5. texts. 2.2 Anaphora type classification This paper is organized as follows. In the next For anaphora type classification, an interesting section, we give a detailed explanation of our in- question is whether this subtask should be carried vestigations. In section 3, we introduce related out before antecedent selection or after. Accord- work. In section 4, the experimental setup is de- ingly, we consider two kinds of configurations: scribed. In section 5, the results of our investiga- Classify-then-Select and Select-then-Classify as tions are shown and discussed. Finally, the conclu- summarized in Figure 1. sion is given and our future work is described. 2.2.1 Classify-then-Select (C/S) model 2 Anaphora Resolution Models Given an anaphor, this model first determines To explore how our two subtasks can be best whether the anaphor bears direct anaphora, indirect configured, we consider several possible models anaphora or exophora. If the anaphora type is clas- for each subtask. sified as direct, then the model calls for the direct 2.1 Antecedent selection antecedent selection model. If classified as indi- For antecedent selection, we can employ a va- rect, on the other hand, then the model calls for the riety of existing machine learning-based methods indirect antecedent selection model. No antecedent designed for coreference resolution ranging from selection model is called for if the anaphora type classification-based models (Soon et al., 2001, is classified as exophora. The model for anaphora etc.) and preference-based models (Ng and Cardie, type classification is trained in a supervised fash- 2001, etc.) to comparison-based models (Iida ion. et al., 2005; Yang et al., 2003, etc.). In this pa- According to the information used for the classi- per, we adopt Iida et al.’s tournament model, a fication, we can consider the following two config- state-of-the-art method for coreference resolution urations: in Japanese (Iida et al., 2005). • a-Classify-then-Select (aC/S) Model: Classify One issue to explore for this subtask is whether the anaphora type by using the anaphor and its a single common model should be trained for both properties before selecting the antecedent. direct and indirect anaphora or whether a separate • c-Classify-then-Select (cC/S) Model: Classify model should be trained for each. So we consider the anaphora type using the anaphor, its prop- the following two approaches: erties and the information from all potential an- • Single model: Train a single model with labeled tecedents before selecting the antecedent. examples of both direct and indirect anaphora. To verify the effects of using additional informa- • Separate model: Train two distinct models sep- tion (i.e., contextual information) we can choose arately for direct and indirect cases; i.e., a direct the aC/S model as a baseline. We give a detail of antecedent selection model is trained only with the feature set for these models in Section 4.2. labeled examples of direct anaphora and an in- 2.2.2 Select-then-Classify (S/C) model direct antecedent selection model only with la- Given an anaphor, this model first selects the beled examples of indirect anaphora. most likely antecedent and then calls for the The separate model approach is expected to be anaphora type classifier to determine the anaphora advantageous because the semantic relation be- type referring to both the anaphor and the selected tween an anaphor and its antecedent tends to be candidate antecedent(s). This way of configura- different in direct and indirect cases. It is crucial to tion has an advantage over the Classify-then-Select identify synonymous relations between an anaphor models in that it determines the anaphora type of a given anaphor taking into account the information The case of the dS/C configuration is slightly of its most likely candidate antecedent. The can- more complex. Analogous to the sS/C model, for didate antecedent selected in the first step can be (7), the classifier takes the CDi paired with its expected to provide contextual information useful direct antecedent albumi as an instance of direct for anaphora type classification: for example, if her anaphora.