Exploring Coreference Features in Heterogeneous Data (2020)

Abstract: The present paper focuses on variation phenomena in coreference chains. We address the hypothesis that the degree of structural variation between chain elements depends on language-specific constraints and preferences and, even more, on the communicative situation of language production. We define coreference features that also include reference to abstract entities and events. These features are inspired through several sources – cognitive parameters, pragmatic factors and typological status. We pay attention to the distributions of these features in a dataset containing English and German texts of spoken and written discourse mode, which can be classified into seven different registers. We apply text classification and feature selection to find out how these variational dimensions (language, mode and register) impact on coreference features. Knowledge on the variation under analysis is valuable for contrastive linguistics, translation studies and multilingual natural language processing (NLP), e.g. machine translation or cross-lingual coreference resolution.

## introduction the way in which coreference is realised in texts is governed by the mode of production, by typical contexts of situation and by language peculiarities. in this study, we are particularly concerned with coreference variation as a result of these three influencing factors. the production and reception of referring expressions in naturally occurring discourse is a reflection of discourse mode (spoken vs. written discourse, see @xcite ). another influence is exerted by discourse genres or registers foot_0 that correspond to standard configurations of communicative topics, goals and speaker interaction, typical of particular discourse communities. we know from register and genre studies (for instance @xcite that register differences can be observed at all linguistic levels and be deduced from lexico-grammatical features. the production and reception of referring expressions is governed by language-specific factors, as coreference relations in different languages vary considerably in the range of linguistic means triggering these relations @xcite @xcite . moreover, there are languagespecific preferences for using particular means over others. variational dimensions such as mode, register and language influence the choice and the frequency of referential expressions in language use. we therefore need to know how this influence is reflected in the kinds of coreference phenomena, their internal organisation (structure) and in their interplay with other related phenomena. apart from answering linguistically motivated contrastive questions, this knowledge is also beneficial to the area of natural language processing, i.e. when designing features for coreference resolution tasks in multilingual heterogeneous data. the importance of the information on this variation is known, as for instance, the conll-2012 shared task on coreference resolution included multiple languages, modes and registers within ontonotes @xcite . information on languagedriven variational mechanisms in coreference is valuable for multilingual coreference resolution systems @xcite @xcite . @xcite describe difficulties and challenges of this task showing that many issues remain unsolved in multilingual coreference resolution. in coreference projection, when the annotation of coreference chains in a source lan-guage is projected onto a target language @xcite @xcite , non-equivalences resulting from language contrasts cause numerous errors. knowledge on the register and on mode differences is also useful for coreference resolution that requires domain adaptation @xcite @xcite . there are studies showing that register and mode impact on anaphora prediction models (see e.g. @xcite . in this paper, we define a number of coreference features that are inspired through several sourcescognitive parameters, pragmatic factors and typological status. we pay attention to the distributions of these features in a dataset containing english and german texts that belong to two different discourse modes (spoken and written) and can be classified into seven different registers (academic speeches, political essays, general interviews, literature, technical manuals, popular science and texts from company websites). as our main goal is to find out how these variational dimensions (language, mode and register) impact on coreference features, we apply data mining techniques focusing on the following research questions (rqs): rq1 which coreference features are most informative in the three prediction tasks: (a) language, (b) mode, (3) register? rq2 which parameters are distinctive for the languages, modes and registers under analysis? ## theoretical background in our study, coreference includes cohesive relations of identity, i.e. relations between coreferring expressions in a text pointing to the same extralinguistic referent. this is illustrated in example (1). the first referring expression in the text is the antecedent (two different people) introducing the referent into the textual world. we account for antecedents referring to referents such as persons, objects, times and locations, and also to more complex semantic concepts such as actions or processes, as in example (2), or facts and events. concepts relating to persons or objects are often expressed by simpler linguistic structures such as noun phrases, while complex concepts are typically reflected by less condensed structures such as sentences and even larger stretches of text and may therefore also function as antecedents in coreference chains. these are also included in our analysis. all other subsequent expressions, referring to the same referent are anaphors -explicit linguistic triggers indicating an anaphoric relation to another stretch of text. they include personal and demonstrative pronouns (it and this), cohesive adverbs of place and time (e.g. here, then) and pronominal adverbs (e.g. herewith in english or damit in german), which are especially frequent in german. they all function as anaphoric heads. moreover we include possessive and demonstrative determiners (these in these two guys in example (1)) and the definite article, functioning as modifiers within the anaphor. the antecedent and all subsequent anaphors pointing to the same referent occur in a coreference chain. in our study, we account for variation of form and structure of coreferring expressions, their grammatical function and syntactic position, as well as variation with respect to the chain relation. most studies on (automatic) anaphora resolution are based on the assumption that the reasons for differences in form, grammatical function and position of coreferring expressions in one and the same chain are related to differences in the degree of accessibility, givenness or salience that a referent has in the recipient's mind at a given point @xcite @xcite @xcite . for instance, coreferring expressions that are realised as pronouns and occur as subjects in sentence-initial position typically signal a high degree of accessibility, whereas full lexical phrases that are non-subjects at sentence-final position typically reflect a lower degree of accessibility. furthermore, the accessibility of a referent is related to chain features (e.g. @xcite : low distance in long coreference chains together with a low general number of different coreference chains is related to a high degree of accessibility. our main interest is functional variation of coreference that mainly stems from three variables of language use -mode of production, register variation and language contrast. we are aware of the fact that these variables interact with the general principles of cognitive processing mentioned above. however, the reflection of the cognitive status in coreference variation itself is not the focus of the current paper. as mentioned above, the range of available and preferred linguistic structures for realising coreference chains differs across languages @xcite @xcite . the two languages under analysis differ in the linguistic forms available to signal coreference: german has more fine-grained options for differentiating degrees of accessibility, such as pronominal adverbs or demonstrative articles, whereas the english language system provides less syntactic flexibility and is more restricted than german in the distribution of accessible or less accessible referents. besides that, english prefers more lexical means for establishing cohesion, whereas german tends to use more grammatical means of coreference @xcite . german also seems to tend towards explicitating coreference relations, especially by using more demonstratives than english. we therefore argue that english and german differ in how correferring expressions vary in their form, syntactic function and position if looking inside coreference chains. for instance, frequent alternations in the use of demonstrative and personal pronouns are common in german, whereas in english, the form of the anaphor generally does not often change.this is illustrated in examples (2) and (3). we work for prosperity and opportunity because they're right. it's the right thing to do. in the english example, the personal pronoun they refers to the entities prosperity and opportunity, and the personal pronoun it -to the event working for prosperity and opportunity. in the translation into german, the demonstrative das and the pronominal adverbial damit refer to the event working for pros-perity and opportunity in both cases. in the second case, an additional logico-semantic relation of instrument is encoded implying a change in terms of form, grammatical function as well as position of the anaphor. high or low variation in the use of different coreference expressions in texts may not only by subject to language contrasts but may also be a reflection of register or/ and the type of language production: example ( 4 ) is an extract from our spoken register interview. it not only shows no variation at all in terms of the form of anaphors used in one coreference chain but concerning their syntactic function and position. moreover, high thematic continuity is reflected by a long coreference chain with small distance between all elements in the coreference chain. these features used in combination typically reflect spontaneous spoken language involving dialogue between at least to speech participants. much more variation can be expected in particular written language registers of our corpus. ## feature categories under analysis our coreference features can be classified into several groups foot_1 . the first group (features 1-24) includes features that are related to the form, to functional and structural properties of coreferring expressions -categories motivated by various pragmatic factors. on the other hand, this is related to the levels of explicitness of coreferential expressions proposed by @xcite who distinguishes three degrees (low, medium and high) of explicitness that rise with the information provided by the referring element. this also goes along with the concept of accessibility of cohesive referents by @xcite -a suitable means to measure coreferential explicitness, although @xcite does not make use of the term explicitness in her work. ## conclusion and discussion we used a set of coreference features of cognitive, pragmatic and typological nature to analyse their variation in heterogeneous data -texts that belong to two different languages, spoken and written modes classified into seven different registers. we used different methods to find out in which way the three variational dimensions that are present in our dataset (language, mode and register) influence the constellation of features. the results show that depending on the variational dimension, we can have different sets of coreference features. the information on the nature of features derived from our analyses can be useful for studies that use heterogeneous datasets for automatic coreference resolution tasks and multilingual coreference projection. depending on the dataset at hand, a feature adaptation is recommended. information on the features that are distinctive for certain classes included into our analysis provide us with patterns of systematic contrasts. the differences in position, grammatical function and forms of coreferring expressions in a source and a target text belonging to the same register cause features distinctive for essay fiction dist-t ant-ttr na na na instr dist-t ant-subj srate3 ant-other ana-pers-mod popsci dist-t ant-ttr ana-dem-local ant-other length web dist-t ant-ttr ant-subj srate4.1 ant-event-vp academic dist-t ant-subj ana-pers-mod ant-ttr ana-obj interview ant-subj ant-obj dist-t ana-pers-mod ana-obj features distinctive for academic essay ana-dem-mod ana-dem-local ana-dem-head ana-pers-it srate4.1 fiction ana-dem-head srate4.1 ana-dem-mod ana-pers-it ant-fact-s instr ana-dem-head ana-dem-local srate3 ana-pers-it srate4.1 popsci ana-dem-head ana-dem-local ant-other ana-pers-it srate4.1 web ana-dem-head srate4.1 ana-dem-local ana-pers-it ant-event-vp interview ana-dem-mod antecedent chain ant-obj ant-subj