Sentence-initial Discourse Connectives, Discourse Structure and Semantics

This paper focusses on differences in the properties of sentence-initial (hereafter, s-initial) coordinating conjunctions (hereafter, CCs) and s-initial discourse adverbials, and what such differences imply for discourse structure and semantics. Data are mainly from the Penn Discourse TreeBank1 (PDTB), which contains the annotation of discourse relations in 2304 articles of the Wall Street Journal corpus (Marcus et al., 1993) in terms of discourse connectives and the minimal text spans that give rise to their arguments (Prasad et al., 2008), as in Example 1: (1) Even so, according to Mr. Salmore, the ad was ”devastating” because it raised questions about Mr. Courter’s credibility. But it’s building on a long tradition. (0041) The connective (“but”) is underlined, the first of its two arguments, ARG1, is shown in italics and the second, ARG2, is shown in boldface. The number 0041 indicates that the example comes from subsection wsj 0041 of the corpus. All annotation has been done by two annotators and adjudicated by a third (often a committee) in cases of disagreement. Only where connectives serve to connect clauses (or possibly an event-denoting nominalization, in the case of ARG1), have they been annotated. The PDTB also annotates the sense of each connective and the attribution of both connectives and their arguments, but since neither plays a role in our presentation here, we omit further discussion. As well as annotating explicit connectives, the PDTB annotates implicit connectives, which the annotators insert between paragraph-internal adjacent sentence not otherwise linked by an explicit connective, in order to express the inferred relation(s) between them. For example, (2) The projects already under construction will increase Las Vegas’s supply of hotel rooms by 11,795, or nearly 20%, to 75,500. By a rule of thumb of 1.5 new jobs for each new hotel room, Clark County will have nearly 18,000 new jobs. (0994) Here, the annotators take the connective “so” as implicitly connecting arguments found in the adjacent sentences, shown in italics (ARG1) and boldface (ARG2). Differences between this style of annotation and annotation in the RST corpus and in the GraphBank corpus are discussed in (Webber, 2006). Here we focus on s-initial discourse connectives whose first argument comes from the preceding discourse (ie, CCs and discourse adverbials). Of interest is the location and form of ARG1, and what it is that cases where ARG1 is not immediately adjacent to the connective say about the intervening matieral. We present data on the CC “but” (as in Example 1) and on “so”, which Huddleston and Pullum (2002) note has more similarities to CCs than it does differences. Both occur frequently in the WSJ corpus as s-initial clause connectors. While ARG1 of these connectives is always to their left and ARG2, to their right, ARG1 is not necessarily immediately adjacent to the connective.2 Specifically, 20% of the 111 instances of s-initial “so” (22) have a non-adjacent ARG1, while 15.4% of s-initial “but” do so. While a similar pattern is found with discourse adverbials like “instead” and “nevertheless”, does this mean that s-initial “but” and “so” behave not as CCs, but as adverbials? Huddleston and Pullum (2002) describe coordinators (including CCs) as expressing “a relation between two or more elements of syntactically equal status”. And in all instances of s-initial “so” and all but one instance of s-initial “but”, we can show that ARG1 spans one or more clauses at a similar level of embedding as ARG2.3 With s-initial discourse adverbials (which Huddleston and Pullum (2002) call “connective adverbials”), this is frequently not the case: Of the 61 s-initial instances of “instead” in the corpus, ARG1 spans text other than the main clause in 31 (51.7%), as in (3) The tension was evident on Wednesday evening during Mr. Nixon’s final banquet toast, normally an opportunity for reciting platitudes about eternal friendship. Instead, Mr. Nixon reminded his host, Chinese President Yang Shangkun, that Americans haven’t forgiven China’s leaders for the military assault of June 3-4 that killed hundreds, and perhaps thousands, of demonstrators (0093).