Semantic Role Labeling as Syntactic Dependency Parsing (2020)

Abstract: We reduce the task of (span-based) PropBank-style semantic role labeling (SRL) to syntactic dependency parsing. Our approach is motivated by our empirical analysis that shows three common syntactic patterns account for over 98% of the SRL annotations for both English and Chinese data. Based on this observation, we present a conversion scheme that packs SRL annotations into dependency tree representations through joint labels that permit highly accurate recovery back to the original format. This representation allows us to train statistical dependency parsers to tackle SRL and achieve competitive performance with the current state of the art. Our findings show the promise of syntactic dependency trees in encoding semantic role relations within their syntactic domain of locality, and point to potential further integration of syntactic methods into semantic role labeling in the future.

## introduction semantic role labeling (srl; @xcite analyzes texts with respect to predicate argument structures such as "who did what to whom, and how, when and where". these generic surface semantic representations provide richer linguistic analysis than syntactic parsing alone and are useful in a wide range of downstream applications including question answering @xcite , open-domain information extraction @xcite , clinical narrative understanding @xcite , automatic summarization @xcite and machine translation @xcite @xcite , among others. it is commonly acknowledged that syntax and semantics are tightly coupled with each other (levin * work done during an internship at bloomberg l.p. she wanted to design the bridge . . . ## syntactic structures of semantic roles it has been widely assumed in linguistic theories that the semantic representations of arguments are closely related to their syntactic positions with respect to the predicates @xcite @xcite @xcite . 2 this notion is articulated as linguistic hypotheses underlying many syntactic theories: from an empirical perspective, most syntactic realizations for semantic arguments follow certain common patterns even when they are displaced. indeed, this is partially why syntax-based features and candidate pruning heuristics have been suc-cessful in srl @xcite @xcite . full parsing might not be necessary to account for the majority of cases in the annotations. thus, knowing the empirical distributions of the arguments' syntactic positions would be highly useful for deciding how detailed the syntactic analysis needs to be for the purpose of srl. in this section, we provide such a characterization. our analysis is based on dependency syntax and complements prior constituent-based characterizations @xcite . one advantage of syntactic dependencies over phrase-structure trees for the purposes of this paper is that the dependents are often more directly connected to the syntactic governors without intervening intermediate constituents. for example, when a verb has multiple adjunct modifiers, each would create an additional intermediate vp constituent in the argument structure, leading to further separation between the verb and the external argument (subject). in contrast, in a dependency representation, the subject is always directly dominated by the verbal predicate. ## experiments we evaluate on two datasets from ontonotes 5.0 @xcite on english and chinese. similar to §2, we adopt the conll 2012 dataset splits. to isolate the effects of predicate identification and following most existing work on srl, we provide our models with pre-identified predicates. we report median performance across 5 runs of different random initialization for our models and our replicated reference models. implementation details are provided in appendix §a. ## conclusion linguistic theories assume a close relationship between the realization of semantic arguments and syntactic configurations. this work provides a detailed analysis of the syntactic structures of propbank-style srl and reveals that three common syntactic patterns account for 98% of annotated srl relations for both english and chinese data. accordingly, we propose to reduce the task of srl to syntactic dependency parsing through back-and-forth conversion to and from a joint label space. experiments show that dependency parsers achieve competitive results on propbank-style srl with the state of the art. this work shows promise of a syntactic treatment of srl and opens up possibilities of applying existing dependency parsing techniques to srl. we invite future research into further integration of syntactic methods into shallow semantic analysis in other languages and other formulations, such as frame-semantic parsing, and other semanticallyoriented tasks. part waiting rcmod the second most common construction involves "vmod" (28%). different from the "vmod" relations involved in (c)-type relations, the non-finite clauses usually modify noun phrases in (r)-type relations. for examples: (7) . . . developed . . . management consultants to go out . . . consultants go vmod (8) the administration, hoping to de-escalate the violence, is appealing to both sides. the other constructions besides the three most common patterns are a mixture of data annotation errors, constituency-to-dependency failures, and combinations of the frequent patterns. if the argument is shared with other predicates along the dependency path, then our conversion algorithm can recover the srl relation through multiple (c)-type labels. for example, in the following sentence, the argument "i" is shared across three predicates "trying", "help" and "fix" as arg0's.