Aspect and Entity Extraction

Abstract

This is the second chapter about aspect-based sentiment analysis. In Section 2.1, we defined each opinion as a quintuple ( e, a, s, h, t ), where e is an entity and a is one of its aspects, s is the sentiment on the aspect a , h is the opinion holder and t is the time when the opinion is expressed. Chapter 5 focused on aspect-based sentiment classification, which determines s . This chapter focuses on extraction of entities and aspects on which sentiments or opinions have been expressed. Entity and aspect extractions are often regarded as two separate tasks because the methods and features used for their recognition are usually different due to their individual specific characteristics. Entities commonly refer to names of products, services, individuals, events, and organizations, and aspects commonly refer to the attributes and components of entities. The two tasks are also collectively called opinion target extraction in sentiment analysis because they together form the targets of opinions. After their extraction, a resolution step is also performed to group synonymous entities and aspects together to facilitate opinion summarization. Let us use two example sentences to ground the tasks: “ I brought a Motorola X phone yesterday, and its voice quality is great .” “ The sound from this Moto X phone is great .” The entities in the two sentences are Motorola X and Moto X , and the aspects are voice quality and sound . The entity and aspect extraction tasks aim to find these entities and aspects. The resolution step should group Motorola X and Moto X together as they refer to the same entity, and voice quality and sound together as they refer to the same aspect. This chapter will not discuss how to determine sentiments on entities or aspects as it has been studied in Chapter 5. In general, both aspect extraction and entity extraction are information extraction tasks. However, in the context of sentiment analysis, some specific characteristics of the problem can facilitate their extractions. A key characteristic is that an opinion always has a target. The target is an aspect or an entity.