Contribution of Linguistic Typology to U niversal D ependency Parsing: An Empirical Investigation (2024)

Abstract: Universal Dependencies (UD) is a global initiative to create a standard annotation for the dependency syntax of human languages. Addressing its deviation from typological principles, this study presents an empirical investigation of a typologically motivated transformation of UD proposed by William Croft. Our findings underscore the significance of the transformations across diverse languages and highlight their advantages and limitations.

## introduction universal dependencies (ud) @xcite is widely used as a standard for morphosyntactic annotations. ever since its initial release in october 2014, however, the scheme has been criticized with respect to its adherence to typological principles @xcite . @xcite cite @xcite 's argument that the nlp community has traditionally had little concern for language typology and linguistic universals. they maintain that the ud initiative, akin to prior parsing and tagging scheme proposals aimed at a universal description of the world's languages, fails to refer explicitly to the extensive typological literature on universals, which accounts for the language-specific annotations that it provides besides those that are actually universal in typological terms. therefore, they continue to propose their own dependency annotation scheme, claiming to represent crosslinguistic variations more comprehensively based on the following four design principles. the first principle distinguishes universal constructions from language-specific strategies and favors classification based on the former. for example, a copula strategy, used in english to realize a predicate nominal construction, may be represented by a different strategy in another language, so the separate relation in ud for copulas is absent in @xcite 's revision. the second principle emphasizes the use of the same labels for the same functions realized syntactically and morphologically. 1 the third principle prioritizes information packaging over lexical semantics and contributes significantly to the provision of a more economic tag set, as in the substitution of the ud relations for different nominal modifiers with a single label, detailed in section 3. the fourth principle emphasizes consideration of dependency structure ranks, including predicates, arguments, modifiers, and adverbs qualifying modifiers, representing a range of dependency levels. this can be instantiated by @xcite 's different treatments of complex sentences, complex predicates, and arguments, although they are all dependent on the predicate. @xcite emphasize that the advantages brought about by their scheme may sacrifice the practical purposes pursued by ud, including achieving high parsing accuracy. this concern has restricted the scheme's application to instructional purposes despite its theoretical potential to address ud's typological gaps. this paper investigates the empirical impact of the scheme on parsing accuracy, aiming to enable its future use in ud revisions. our results on a typologically diverse set of languages confirm that it is more straightforward to parse treebanks with typologically informed ud annotation (referred to as tud henceforth) than to parse ones with standard ud annotation. the results show significant but not necessarily fundamental improvement, as @xcite 's proposals address only the classification of dependency relations without affecting the overall tree structure. ## related work incorporating knowledge of language diversity into nlp systems is widely regarded as a valuable strat-egy for enhancing language independence @xcite . in the context of universal dependencies, the literature addresses typological limitations through parsing architecture and annotation scheme considerations. @xcite integrate the notion of syntactic nuclei into the ud parsing framework to cope with the typological differences of languages. their experimentation demonstrates that nucleus composition consistently improves parsing accuracy. this idea is further explored by @xcite , who find that the observed parsing improvement results from the greater capability of the enriched models of analyzing main predicates, nominal dependents, clausal dependents, and coordination structures. other proposals present alternative annotation schemes or revisions to ud. @xcite propose the surface-syntactic universal dependencies (sud), claimed to be a richer and easier variant of ud. they argue that sud treebanks enable cross-linguistic typological measures thanks to their distributional and functional criteria. @xcite recall the sud's general principles, update its relation set, address annotation issues, and present an orthogonal layer of syntactic features. @xcite further suggest that a new treebank should initially be developed in sud, even if a ud treebank is intended. the 2021 international conference on parsing technologies @xcite was dedicated to the additional structural layer of ud, known as enhanced universal dependencies (eud), to encode grammatical relations that can be represented more adequately using graphical rather than purely rooted trees. this paper examines a typologically revised annotation scheme for ud, called tud, based @xcite 's proposal. unlike sud and eud, which modify dependencies structurally, tud affects only the dependency labels while preserving the dependency tree topology. furthermore, it involves less radical dependency relation mappings and retains the majority of original ud labels regardless of the corresponding pos tags. ## transformation we devise a set of transformation rules in the form @xmath0. @xcite distinguish the subject relation from object and oblique. they label this relation sbj regardless of its categorization as a noun phrase or a clause, in line with their fourth principle. this is realized in our script via the consolidation rules @xmath1. furthermore, they find it redundant under the third principle to tag direct and indirect objects differently, so we consider consolidation rules @xmath2. the asterisk indicates that obj is already a ud relation, with the latter rule assumed to retain it throughout the conversion. @xcite challenge the distinction made in ud between complements in terms of grammatical role, including obligatory and nonobligatory control. our consolidation rules @xmath3, conforming to the third principle. moreover, they point out that ud treats resultatives as controlled complements, which it labels xcomp. they suggest that these complex predicate elements be labeled similarly to other secondary predicates and adverbs of manner, which are tagged sec. the rule @xmath4, complying with the fourth principle. thus, the fragmentation rules @xmath5. @xmath6, which is assumed not to be the case for resultatives, where @xmath7. ud treebanks optionally set the morphological feature advtype with different values for adverbs of manner, location, time, quantity or degree, cause, and modal nature. on the other hand, @xcite propose in line with their fourth principle that the diversity of adverbs in semantics, syntactic distribution, and morphological form needs to be captured and suggest that adverbs of manner should be labeled sec, and ones expressing degree or hedging, aspect or modality, and location or time should be tagged qlfy, aux, and obl, respectively. therefore, the fragmentation rules @xmath8. according to the ud documentation, the major values include man, for adverb of manner, loc, for adverb of location, tim, for adverb of time, deg, for adverb of quantity or degree, cau, for adverb of cause, and mod, for adverb of modal nature. where a different or no setting exists, @xmath9, as @xcite assert that the ud advmod relation should be excluded altogether. predicates, tagged cxp, unlike in ud, where they are treated similarly to nominal compounds. therefore, the rule @xmath10, in accordance with the fourth principle, to transform the ud compound relation to cxp where the token's parent is pos-tagged verb, assumed to signal a light verb construction alongside the token's own compound dependency relation label. they also suggest that copulas should be treated as light verbs, hence the consolidation rule @xmath11, which conforms to the first principle. furthermore, they suggest that nummod, amod, and det should all be tagged mod, as they involve the same type of information in general, conforming to the third principle. the consolidation rules num-@xmath12, @xmath13, and @xmath14. figure 1 summarizes the transformations. it should be noted that the eventual aim of this paper is to pave the way for the creation of a totally typologically-based version of ud. the intended scheme will be applicable as a basis for the annotation of text from scratch, involving all the considerations made in @xcite . since that would be a costly transformation, we need to ensure beforehand that it merits the cost. therefore, we attempt a preliminary transformation phase, where we apply changes to the available ud treebanks under the limitations imposed by the ud guidelines. in other words, the treebanks resulting from the conversion procedure are intermediary means that enable empirical investigation rather than finalized corpora prepared for use by a corpus linguist. we provide a manual evaluation of the proposed transformation in the next section. ## experiments and results we evaluate the impact of the typological transformations based on their contribution to parsing performance. our test benchmark consists of 20 treebanks from ud 2.12 belonging to diverse language families, inspired by @xcite . in addition to language diversity, we consider the presence of labels needed for the maximal application of the transformation rules. for this purpose, we incorporate treebanks that include the annotations required for the transformation. as stated in section 3, for instance, the morphological feature annotation on adverb types, required for our transformation of the advmod relation, is optional according to the ud guidelines. therefore, we add some of the few languages that have included this information in order to cover that specific transformation. ## conclusion the typological transformation of universal dependencies presents an advantage in terms of parsing performance. this benefit is observable across the two primary parsing approaches, namely the transition-based and the graph-based parsing, and in many languages. the positive impact on parsing performance can be attributed to the consolidation rules, which merge the dependency relations with similar typological properties. on the contrary, the parsing performance is slightly hindered by fragmentation rules, indicating their detrimental effect in the context of universal dependencies. our empirical results demonstrate that an annotation scheme resulting from the typological transformation does not sacrifice the practical aims of ud. therefore, we suggest establishing such a scheme as an alternative basis for treebanking. our manual evaluation highlights the importance of typological annotation from scratch or the use of more advanced automatic conversion from the existing ud scheme. in future work, we plan to improve the conversion method and explore the practical benefits of the proposed scheme in downstream tasks.