1 Topical overview

Knowledge graphs are an effective way to store semantics in a structured format that is easily used by computer systems. In the past few decades, work across different research communities led to scalable knowledge acquisition techniques for building large-scale knowledge graphs. The result is the emergence of large publicly available knowledge graphs (KGs) such as DBpedia (Lehmann et al. 2014), Freebase (Bollacker et al. 2008), and others. While knowledge graphs are designed to support a wide set of different applications, this special issue focuses on the use case of text retrieval and analysis.

Utilizing knowledge graphs for text analysis requires effective alignment techniques that associate segments of unstructured text with entries in the knowledge graph, for example using entity extraction and linking algorithms (Carmel et al. 2014; Mendes et al. 2011; Blanco et al. 2015). A wide range of approaches that combine query-document representations and machine learning repeatedly demonstrate significant improvements for such tasks across diverse domains (Dalton et al. 2014; Liu and Fang 2015; Hasibi et al. 2015; Xiong and Callan 2015; Raviv et al. 2016; Ensan and Bagheri 2017; Xiong et al. 2017). The goal of this special issue is to summarize recent progress in research and practice in constructing, grounding, and utilizing knowledge graphs and similar semantic resources for text retrieval and analysis applications. The scope includes acquisition, alignment, and utilization of knowledge graphs and other semantic resources for the purpose of optimizing end-to-end performance of information retrieval systems.

2 Articles in this special issue

Extraction errors, missing relations, and yet unobserved entity-facts lead to never-complete knowledge graphs. Entity linking and missing link prediction methods make use of entity set expansion as an underlying component. Rastogi et al. (2018) suggest a neural auto-encoder framework for entity set expansion, which particularly excels in difficult situations when entities appear in multiple sentences and many entities are contained in the query.

For similar reasons, large knowledge graphs are undergo continuous improvement cycles. With the increased coverage of entities and facts it is likely that the ontological type structure needs to be modified. To compare and assess different versions of an evolving knowledge graph, Nayak et al. (2018) develop a similarity measure for knowledge hierarchies.

Several works focus on search queries that ask for entities. Garigliotti et al. (2018) focus on how to best exploit type information that is present queries for entity retrieval. Different approaches to matching type information to entities are explored ranging between top-level types and most specific types in the hierarchy.

Sawant et al. (2019) develop a method to exploit entity mentions and type hints to reason about the target entity. The approach combines information from a knowledge graph with information from a full text corpus with entity link annotations. These heterogeneous pieces of information are integrated through multiple convolutional networks to gracefully handle cases where exact word matches fail.

Jimmy et al. (2018) explore KG-aware retrieval models in the context of consumer health search. A key challenge of the consumer health domain is the vocabulary mismatch problem. To this end, Jimmy et al. (2018) using knowledge graphs to match consumer queries to the medical terminology used in documents.

Addressing the population of knowledge graphs with text, where each entity is associated with several facets (i.e., headings on Wikipedia pages), MacAvaney et al. (2018) suggest an approach for identifying relevant text passages. Where a traditional information retrieval approach would include such facets into the keyword query, MacAvaney et al. (2018) address the problem of low-utility facets, these are frequent headings whose words are unlikely to appear in relevant passages.

These six contributions discuss important aspects that arise when knowledge graphs are populated and used in the context of information retrieval. Each of them makes a strong case for integrating unstructured text and structured knowledge across a wide range of domains and applications.

3 Selection process and community

For this special issue we selected six articles out of 23 submissions. Each article was reviewed by at least three reviewers and underwent at least one revision.

More literature on how to effectively use of knowledge graphs in information retrieval can be found in the proceedings of the KG4IR Workshop series. Inqueries about methods and topical discussions can be directed to the discussion forum kg4ir@googlegroups.com.