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<Paper uid="W06-0306">
  <Title>Searching for Sentences Expressing Opinions by using Declaratively Subjective Clues</Title>
  <Section position="3" start_page="0" end_page="39" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Readers have an increasing number of opportunities to read opinions (personal ideas or beliefs), feelings (mental states), and sentiments (positive or negative judgments) that have been written or posted on web pages such as review sites, personal web sites, blogs, and BBSes. Such subjective information on the web can often be a useful basis for finding out what people think about a particular topic or making a decision.</Paragraph>
    <Paragraph position="1"> A number of studies on automatically extracting and analyzing product reviews or reputations on the web have been conducted (Dave et al., 2003; Morinaga et al., 2002; Nasukawa and Yi, 2003; Tateishi et al., 2004; Kobayashi et al., 2004). These studies focus on using sentiment analysis to extract positive or negative information about a particular product. Different kinds of subjective information, such as neutral opinions, requests, and judgments, which are not explicitly associated with positive/negative assessments, have not often been considered in previous work. Although sentiments provide useful information, opinion-expressing sentences like &amp;quot;In my opinion this product should be priced around $15,&amp;quot; which do not express explicitly positive or negative judgments (unlike sentiments) can also be informative for a user who wants to know others' opinions about a product. When a user wants to collect opinions about an event, project, or social phenomenon, requests and judgments can be useful as well as sentiments. With open-domain topics, sentences expressing sentiments should not be searched exclusively; other kinds of opinion expressing sentences should be searched as well.</Paragraph>
    <Paragraph position="2"> The goal of our research is to achieve a web search engine that locates opinion-expressing sentences about open-domain topics on products, persons, events, projects, and social phenomena.</Paragraph>
    <Paragraph position="3"> Sentence-level subjectivity/objectivity classification in some of the previous research (Riloff and Wiebe, 2003; Wiebe and Riloff, 2005) can identify subjective statements that include speculation in addition to positive/negative evaluations. In these efforts, the subjectivity/objectivity of a current sentence is judged based on the existence of subjective/objective clues in both the sentence itself and the neighboring sentences. The subjective clues, some adjective, some noun, and some verb phrases, as well as other collocations, are learned from corpora (Wiebe, 2000; Wiebe et al., 2001). Some of the clues express subjective meaning unrestricted to positive/negative measurements. The sentence-level subjectivity ap- null proach suggests a way of searching for opinion expressing sentences in the open domain.</Paragraph>
    <Paragraph position="4"> The problem of applying sentence-level subjectivity classification to opinion-expressing sentence searches is the likelihood of collecting too many sentences for a user to read. According to the work of Wiebe et al. (2001), 70% of sentences in opinion-expressing articles like editorials and 44% of sentences in non-opinion expressing articles like news reports were judged to be subjective. In analyzing opinions (Cardie et al., 2003; Wilson et al., 2004), judging document-level subjectivity (Pang et al., 2002; Turney, 2002), and answering opinion questions (Cardie et al., 2003; Yu and Hatzivassiloglou, 2003), the output of a sentence-level subjectivity classification can be used without modification.</Paragraph>
    <Paragraph position="5"> However, in searching opinion-expressing sentences, it is necessary to designate criteria for opinion-expressing sentences that limit the number of retrieved sentences so that a user can survey them without difficulty. While it is difficult to formally define an opinion, it is possible to practically tailor the definition of an opinion to the purpose of the application (Kim and Hovy, 2004).</Paragraph>
    <Paragraph position="6"> This study introduces the notion of declaratively subjective clues as a criterion for judging whether a sentence expresses an opinion and proposes a method for finding opinion-expressing sentences that uses these clues. Declaratively subjective clues such as the subjective predicate part of the main clause and subjective sentential adverb phrases suggest that the writer is the source of the opinion. We hypothesize that a user of such an &amp;quot;opinion-expressing sentence&amp;quot; search wants to read the writer's opinions and that explicitly stated opinions are preferred over quoted or implicational opinions. We suppose that writer's ideas or beliefs are explicitly declared in a sentence with declaratively subjective clues whereas sentences without declaratively subjective clues mainly describe things. The number of sentences with declaratively subjective clues is estimated to be less than the number of subjective sentences defined in the previous work. We expect that the opinion expressing sentences identified with our method will be appropriate from the both qualitative and quantitative viewpoints.</Paragraph>
    <Paragraph position="7"> Section 2 describes declaratively subjective clues and explains how we collected them from opinion-expressing sentences on Japanese web pages retrieved with opinion search queries. Section 3 explains our strategy for searching opinion-expressing sentences by using declaratively subjective clues. Section 4 evaluates the proposed method and shows how the opinion-expressing sentences found by the proposed method are congruent with the sentences judged by humans to be opinions.</Paragraph>
  </Section>
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