Leveraging Entities in Document Retrieval

Abstract

This chapter focuses on the classic IR task of ad hoc document retrieval and discusses how entities may be leveraged to improve retrieval performance. At their core, all document retrieval methods compare query and document representations. Traditionally, these representations are based on terms (words). Entities facilitate a semantic understanding of both the user's information need, as expressed by the keyword query, and of the document's content. Entities thus may be used to improve query and/or document representations. As a first step of that process, entities that are related to the query need to be identified, thereby establishing a mapping between queries and entities. As we shall explain in Sect. 8.1, one may go beyond considering only those entities that are explicitly mentioned in the query. We next present three different families of approaches, which are illustrated in Fig. 8.1. • Expansion-based methods utilize entities as a source of expansion terms to enrich the representation of the query (Sect. 8.2). • Projection-based methods treat entities as a latent layer, while leaving the original document/query representations intact (Sect. 8.3). • Entity-based methods consider explicitly the entities that are recognized in documents, as first-class citizens, and embrace entity-based representations in "duet" with traditional term-based representations in the retrieval model (Sect. 8.4). This particular order corresponds to the temporal evolution of research in this area, where the tendency toward more and more explicit entity semantics is clearly reflected. Throughout this chapter, we shall assume that both queries and documents have been annotated with entities, using entity linking techniques we have discussed before (see Chap. 5 for documents and Sect. 7.3 for queries). A particular challenge involved here is how to deal with the uncertainty of these automatic annotations. In practice, necessitated by efficiency considerations, all methods described in this chapter are implemented as re-ranking mechanisms. The details are found in Sect. 8.5. Finally, we present standard datasets and useful resources in Sect. 8.6. We refer to Table 8.1 for the notation used throughout this chapter.