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<Paper uid="C88-1070">
  <Title>Schema Method: A Framework for Correcting Grammatically Ill-formed Input</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1deg Introduction
</SectionTitle>
    <Paragraph position="0"> Ill-formed input cannot be ignored when a natural language processing system such as a computer assisted instruction (CAD system or a machine translation system is built. Particularly in a CAI System, students often make mistakes, such as mispunctuation, lack of agreement, misplaced/improperly-used words, etc. In these cases, a CAI system needs to point out input errors, and show why the input it~ wrong. In order to do so, the system needs to diagnose and correct ill-formed input to explain the errors.</Paragraph>
    <Paragraph position="1"> The schema method as a framework for correcting grammatically ill-formed input is suggested and the diagnosis and correction of errors is discussed.</Paragraph>
    <Paragraph position="2"> There have been many studies for processing ill-formed input for English. The point of those studi.es is the diagnosis: how does the system find an error? The approaches are classified into two groups: the syntactically-oriented group and the frame-based group.</Paragraph>
    <Paragraph position="3"> The syntactically-oriented group includes robust parsers based on Augmented Transition Networks (ATN) which Use the relaxation technique/Kwansny 1981./or the meta-rule/Weisehedel 1980, 82, 87/, and the EPISTLE system which addresses the problems of the checking grammar and style of texts, such as letters, reports and manuals, written in ordinary English/Heidorn 1982/, /Jensen 1983/.</Paragraph>
    <Paragraph position="4"> The frame-based group attempts to deal with ungramnmtical input through extensions to pattern matching parsing/Hayes 1981/, through conceptual case frame instantiation/Schank 1980/and through approaches involving multiple cooperating parsing strategies /Carbonell 1983/. The target of that study is dialogue phenomena in communication with limited-domain systems, such as data-base systems, electronic mail systems, etc.</Paragraph>
    <Paragraph position="5"> The aim of this study is error-correction of non-native speakers written English text. This approach is syntactically oriented.</Paragraph>
    <Paragraph position="6"> The syntactically-oriented approaches/Kwansny 1981/ /Weischedel 1980,82,87/,/Heidorn 1982/,/Jensen 1983/are very similar. Their basic idea is relaxation. They first attempt to parse the input, using fully grammatical rules. If the sentence is not parsed, some of the conditions are relaxed. However these approaches have two major drawback.</Paragraph>
    <Paragraph position="7"> (1)Relaxation control strategies: when inputs are illformed, some means of ranking alternatives is appropriate. The number of relaxed configurations may be large.</Paragraph>
    <Paragraph position="8"> One of the most critical problems is control. The need to relax the very rules that constrain the search for an interpretation is like opening Pandora's box./Weischedel 1987(PP.117)/ (2)Computational inefficiency: the relaxation approach cannot recognize ill-formed input before the analysis with well-formed grammar is finished. Furthermore, fully well-formed grammar is needed. To make fully well-formed grammar, subcategorization of parts of speech is needed and other conditions are added. As a result, there are too many rules.</Paragraph>
    <Paragraph position="9"> In comparison to previous approaches, this approach does not use the relaxation technique. The difference between previous approaches and this one is the method of recognizing an ill-formed sentence. Previous approaches first use a strong filter, then relax the conditions. This approach, however, first uses weak grammars, and then strongly filters the passed sentence. This approach recognizes a sentence at two steps.</Paragraph>
    <Paragraph position="10"> An attempt is made to expand lexical-functional grammar (LFG) /Kaplan 1982/to deal with ill-formed input. LFG has two drawbacks: (1) LFG can't deal with errors of omission and (2) LFG has no framework for error correction. If an input sentence is well-formed, this framework obtains an LFG f-structure. If not, the sentence is corrected.</Paragraph>
    <Paragraph position="11"> Examples of error correction are given in the next section. In the section following the basic idea is described 3~i and the problem of a unification mechanism for processing ill-formed input is discussed. This framework is shown in section 4.</Paragraph>
  </Section>
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