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<?xml version="1.0" standalone="yes"?> <Paper uid="C88-2092"> <Title>Why Computational Grammarians Can Be Skeptical About Existing Linguistic Theories</Title> <Section position="1" start_page="0" end_page="449" type="abstr"> <SectionTitle> PANEL </SectionTitle> <Paragraph position="0"> The bottle neck in building a practical natural language processing system is not those problems which have been often discussed in research papers, but in ilandling much more dirty, exceptional (for theoreticians, but we frequently encounter) expressions. This panel will focus on the problem which has been rarely written but has been argued informally among researchers who have tried to build a practical natural language processing system at least once.</Paragraph> <Paragraph position="1"> Theory is important and valuable for the explanation and understanding, but is essentially the first order approximation of a target object. As for language~ current theories are Just for the basic part of the language structure. Real language usage is quite different from the basic language structure and a supposed mechanism of interpretation. Natural language processing system must cover real language usage as much as possible. The system model must be designed in such a way that it is clearly understandable by the support of a powerful linguistic theory, and still can accept varieties of exceptional linguistic phenomena which the theory is difficult to treat. How we can design such a system is a major problem in natural language processing, especially for machine translation between the languages of different linguistic families. We have to be concerned with both linguistic and non-llngulstlc world. While we have to study these difficult problems, we must not forget about the realizability of a useful system from the standpoint of engineering.</Paragraph> <Paragraph position="2"> I received valuable comments from Dr. Karen Jensen who cannot participate in our panel, and kindly offered me to use her comments freely in our panel. I want to cite her comments in the followings.</Paragraph> <Paragraph position="3"> i. We need to deal with huge amounts of data (number of 5. We are not interested in using the most constrained/ sentences, paragraphs, etc.). Existing linguistic restricted formalism. LTs generally are, because of theories (LTs) play with small amounts of data.</Paragraph> <Paragraph position="4"> 2. The data involve many (and messy) details. LTs are prematurely fond of simplicity. For example: punctuation is very important for processing real text, but LTs have nothing to say about it. (This is actually strange, since punctuation represents -- to some extent -- intonational contours, and these are certainly linguistically significant.) 3. There is no accepted criterion for when to abandon an LT; one can always modify theory to fit counterexamples. We have fairly clear criteria: if a computational system cannot do its Job in real time, then it fails.</Paragraph> <Paragraph position="5"> 4. We need to use complex attribute-value strnctures, which cannot be manipulated on paper or on a blackboard. &quot;Trees&quot; are only superficially involved. This means we are absolutely committed to computation.</Paragraph> <Paragraph position="6"> LTs have various degrees of commitment.</Paragraph> <Paragraph position="7"> Existing linguistic theories ate of limited usefulness to broad-coverage, real-world computational grammars, perhaps largely because existing theorists focus on limited notions of &quot;grammaticality,&quot; rather than on the goal of dealing, in some fashion, with any piece of input text. Therefore, existing theories play the game of ruling out many strings of a language, rather than the game of trying to assign plausible structures to all strings. We suggest that the proper goal of a working computational grammar is not to accept or reject strings, but to assign the most reasonable structure to every input string, and to comment on it, when necessary. (This goal does not seem to be psychologically implausible for human beings, either.) For years it has seemed theoretically sound to assume that the proper business of a grammar is to describe all of the grammatical structures of its language, and only those stmctrees that ate granlmatical: The grammar of L will thus be a device that generates all of the grammatical sequences of L and none of rhe ungrammatical ones. (Chomsky 1957,</Paragraph> <Paragraph position="9"> supposed claims about language processing mechanismsdeg 6. We are interested in uniqueness as much as in generality. ITs usually are not.</Paragraph> <Paragraph position="10"> 7. We are more interested in coverage of the gran~ar than in completenesslof the grammar. LTs generally pursue completeness.</Paragraph> <Paragraph position="11"> 8. We aim for &quot;all,&quot; but not &quot;only&quot; the grammatical constructions of n natural language. Defining ungrammatical structures is, by and large, a futile task (Alexis Manaster-Ramer~ Wlodzimierz Zadrozny).</Paragraph> <Paragraph position="12"> 9. Existing LTs give at besta high-level specification of the structure of natural language. Writing a computational granmmr is llke writing a real program given very abstract specs (Nelson Uorrea).</Paragraph> <Paragraph position="13"> i0. We are not skeptical of theory, Just of existing theories.</Paragraph> <Paragraph position="14"> At first blush, it seems unnecessary to conjure up any justification for titis claim. Almost by definition, the proper business of a grammar should be grammaticality. However, it has been notoriously difficult to draw a line between &quot;gram. maticai&quot; sequences and &quot;ungnmunalicai&quot; sequences, for any natural human language. It may even be provably impossible to define precisely rhe notion of grammaticality for any language. Nalural language deals with vague predicatus, and might itself be called a vague predicator.</Paragraph> <Paragraph position="15"> This being tree, it still seems worthwhile to ~ at parsing ALL of the gr,'unmalical strings of a language, but parsing ONLY the grammatical strings becomes a dubious enteq~rise at best. Arguments for doing so reduce either to dogma, or to some general notion of proptiety. Argmnenis against, however, arc easy to come by. Leaving theoretical considerations aside for the moment, consider these praguratic ones: (a) The diachronic argumeut. The creativity of human use of language is great, and language systems are always changing. A construction that was once unacceptable becomes acceptable over time, and vice versa. Even if a grammar could d~:seribe at |and only file g~armnatical .sequences today, the t~ane may uot be tree tomon'ow. So there is, at best, only an ~u:ademic iuterest in only-g~nmuaticai stmctul'es.</Paragraph> <Paragraph position="16"> (b) The ptacrical argumeut. In tile area of alpplied com null p~ltational linguistics, ill-formeti input is a part Of daily life, a~ld u working gmlmuar has to Ilandie it. By &quot;handle it&quot; we ~'Leau no~. grind to l* ilalt, but figu~ out some kind of appropriate ana|ysls and then comment, if possible~ on whatever is d~fticnlt or mnmual, it' real-lit'e natural language processing i~: gnltig to c~ist, the~ must be anme way to exla~t meaning e~en t~xa'~ s|dnga that violate cnstommy syntactic mien, that a,e exc,.:ssiveiy ~oug and complex, and that are net sentences \[J~: ,-ill.</Paragraph> <Paragraph position="17"> At ~BM Re, arch, we are developing a broad-coverage l~.ar~,de.g granlmar for English, called tile PLNLP EnglLsh ~ralnmar, og PEG. Its initial sylUactic component works only with limited infomratiou - lexical featm~es for pails of speech, for mo~phologic~d stmelme, and for some valeucy classes. This colnpunetlt tries to assilpl some n~asonable st~xtcture to any input siting of English.</Paragraph> <Paragraph position="18"> Even iu its Cnrk~3,nt be~iflntng 8tale, PEG has proved to be ~t' considerable ~sefolness for a lather wide valiety of real-world kWLP taskz. Its main use so fitr has been as the pin, lug C31kLpolleut of CRITIQUE. a large-scale natural language text pr~gx~s'ding systetn that identifies grammar and style errors in ~-;ll,tglis|i text (Iqeido~u et at. i982, Richatdann and Bradeuk\[mdC/~' 1988). A pt-ototype cxrrlQuE system is UOW fmLct~mling hi thr~ major ,'qtplicatitm areas: business offices, a paldislliug center aLrd univeaalries.</Paragraph> <Paragraph position="19"> Real-world natutM language processing nmst deal with huge amounts of data, which involve many, and messy, details, lf~or example, ~mnctuallon is very impmlant in processing real t!~X~, but cm~nt liuguistic theorios have nothing substantial to say about puuctuatinlL. Nor have they anything substantial to ~ay about \[aialysls slructures for ellipsis, or for strings that deviate ill various degrees frmn tim canmiical order of the l;mguage ill which they OCCllr. Here is the kinti of natural language ilqmt that CRITIQUI?; has to deal wilh. (All of tile text excerpts below are wrilten EXAC~I'LY ~Ls they were produced.) Fixzt, a memo that was sent via electronic matt to multiple r, sers ill the of/ice envil'onnLe|tt: (1) Over tile comse of tile next couple of days tile accouoting (lepartment will cooducting inventory of labs and offices here at X~-L-~X. I they are currently workiug on file tirst floor, ~unl woddng tirere way niL.</Paragraph> <Paragraph position="20"> If you are ilOt hi yore&quot; oflice and do not plan to be there withiu the uext few days,please secme all conlidcntial mail tuLd items you may have of confidential ualuL'e. Because if you .are LInt tiLere accontlting is going to go iu and inventory your equipment.</Paragraph> <Paragraph position="21"> Tile author of text (1) is a ualive speaker of American Euglish, wile ilas a college edncatimi and is employed in a .position of some responsibility ill a large business firm. Note 2ire following problems: (a) &quot;will Collducting&quot; should t~ &quot;will conduct&quot;; (b) &quot;conducting inventory&quot; should be &quot;onuducting an i*wentory&quot;; (c) &quot;l 6~ey&quot; should he just '&quot;l~ey&quot;; (d) &quot;worl~ing the~e way up&quot; shmdd be &quot;working their way up'; (e) &quot;days,please&quot; lacks a space hetweeu tim comma and &quot;please&quot;; (i) &quot;of confidential nature&quot; wo~ld be better written ~'; &quot;of a conridential nature&quot;; (g) The last text segmeut is a fragment, not a com null plete clause, although it is p~esented as if it were i~ seutetlC~,.</Paragraph> <Paragraph position="22"> No liieo~eticaliy pure grammar wmtid ever be able to ann * ,dyz, e text like this. It may be objected that &quot;granlmar&quot; defines tire competeuce thai makes it possible for us to identify mist~es (a - g), aml that any working system is an embodlmeot of a kind of performauce, not competence. Very well; note then that the role of &quot;gra~nmar&quot; becomes that of a COMMEbrYARY ou tile analysis strnctuce, NOT the definition of tile structure itself. This is exactly the point. It may be timt we need a new defiuiiiou of tile teri~l &quot;gra1~mar.&quot; Within the educational environment, the ch~dienge for a computational gmmmtn&quot; is even stronger. Followhig are two excerpts from essays by non-native English speakers. Text (2) is an extreme exanLpte of tile ron,-on style of writing; the interesting &quot;grammatical&quot; question is what cues might be used to divide this text into separate sentences: (2) After the analysis of three graphs we can make conclusion. From 1940 to 1980 the farm popstation and farms decrease but the average farm siz~e increase, this tendency shows American don't have strong intensie to work on the farms, as a result it is impossible to increase the farms but when The people who would like to work ou farms expand their f,'um size by themselves or the aid of government; maybe some other agents want to invest capital in tim &quot;farming industry&quot;.</Paragraph> <Paragraph position="23"> Text (3) shows interesting problems with tile definite article (mass vs. count NPs) and with auxiliaries in VPs: (3) So we know, now we can use tile fewer peo- null ple to get the more food. Is the decreasing farmer we deduce on tile graph7 Is the farms going to tiecreasing in luture7 Does the average of farm size will develope7 No. No. No.</Paragraph> <Paragraph position="24"> The problem of non-&quot;grammaticality&quot; is pervasive hi real language use. The question (4) Who did you tell me that won? supposedly poses an cxtraclinu problem - in terms of Government Bhtding Theory, it violates tire Empty Case Principle. Yet it can be heard from the mouths of people who wonkl o111etwlse qualify as speakers of Standard English. The sentence (5) He bought for ten shillings a ring.</Paragraph> <Paragraph position="25"> supposedly violates an ordering constraint ill English because the prepositional phrase &quot;for ten shillings&quot; precedes the direct object &quot;a ring.'~ However, as the direct object NP becomes heavier and heavier, the sentence sounds better and better: (5') He bought for ten shillings a ring that derighted the woman who had previously been proposed to by millionaires.</Paragraph> <Paragraph position="26"> To move &quot;for ten shillings&quot; to a position following tim direct object in (5') would be extremely awkward. Ill this case, it is better to interpret tile &quot;granrmalical&quot; ordering role as a stylistic commem. The consm~otion (6) Hlmself's father came.</Paragraph> <Paragraph position="27"> violates theoretical restrictions on aaaphora, or Binding; but it is fine if mad with an Irish flavor. And the alternative of having a completely separate grammar for Irish English is not appealing. The sentence (7) Site be happy.</Paragraph> <Paragraph position="28"> is censured because the main verb is not tensed; but (7) is valid Non-standard Black English. And so on. Many theoretically proscribed sequences exist and flourish as stylistic or social variants. To ignore them, and to l~rsue the Holy Grail of a grammar that describes &quot;all and only&quot; the grammatical strings of a language, would be to defeat the enterprise of broad-coverage computational parsing.</Paragraph> <Paragraph position="29"> Furthermore, it is not uecessary to enforce all of the supposedly &quot;grammatical&quot; restrictions within a computational analysis grammar that actually deals with quantities of real text, fil real flare. Our experience with PEG, in rile CRITIQUE application, proves this. PEG produces appropriate parses for (4) - (7). Then a Style component can comment on the parses, calling attention to whatever problems or variations exist. We do not cut~ntly handle all of the difficulties posed by (1) (3), but we do handle some of them. For those grammatical restrictions that have to be enforced within the syntactic grammat' (such as number agreement), we have a two-pass error detection and co ffecrion strategy. For massive problems like the nm-ons in (2), we use the technique of the &quot;fitted parse,&quot; which tries to identify sensible chunks of text and present them ill genre reasonable framework.</Paragraph> <Paragraph position="30"> Since it is neither desirable nor necessary for a compuo tational grammar to define &quot;all and only&quot; the &quot;grammatical&quot; sequences of a language, and since working computational grammars are the most comprehensive descriptions that we can come up with, right now, for natural languages, we suggest that the goal of real-world granlmatical analysis be re-defined: a grammar should try to describe &quot;all,&quot; but not &quot;only,&quot; the grammatical strings of a language.</Paragraph> </Section> class="xml-element"></Paper>