The Penn Discourse Treebank: An Annotated Corpus of Discourse Relations

Understanding discourse relies to a great extent on correctly interpreting relations holding between the eventualities and facts mentioned in discourse. These discourse relations, such as causal, contrastive and temporal relations, can be expressed explicitly or implicitly in the discourse, and are the subject of annotation in the Penn Discourse Treebank (PDTB). This chapter presents a case study of the PDTB. Starting with the main ideas behind the annotation framework, we provide a brief overview of the annotation and representation, describe the research and other annotation efforts that the corpus has led to, and finally discuss some major challenges that have arisen in annotating the PDTB, focusing in particular on the problem of characterizing and identifying, via annotation, explicit as well as implicit signals of discourse relations, and of designing the overall annotation workflow.