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<Paper uid="E83-1011">
  <Title>AN EXPERIMENT WITH HEURISTIC PARSING OF SWEDISH</Title>
  <Section position="3" start_page="0" end_page="66" type="intro">
    <SectionTitle>
I BACKGROUND
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    <Paragraph position="0"> The genera ! idea behind the system for heuristic parsing now being developed at our group in Stockholm dates more than 15 years back, when I was making an investigation (together with Hans Karlgren, Stockholm) of the possibilities of using computers for information retrieval purposes for the Swedish Governmental Board for Rationalization (Statskontoret). In the course of this investigation we performed some psycholingulstic experiments aimed at finding out to what extent surface markers, such as endings, prepositions, conjunctions and other (bound) elements from typically closed categories of linguistic units, could serve as a base for a syntactic analysis of sentences. We sampled a couple of texts more or less at random and prepared them in such a way that stems of nouns, adjectives and (main) verbs these categories being thought of as the main carriers of semantic Information - were substituted for by a mere &amp;quot;-&amp;quot;, whereas other formatives were left in their original shape and place. These transformed texts were presented to subjects who were asked to fill in the gaps in such a way that the texts thus obtained were to be both syntactically correct and reasonably coherent.</Paragraph>
    <Paragraph position="1"> The result of the experiment was rather astonishing. It turned out that not only were the syntactic structures mainly restored, in some few cases also the original content was reestablished, almost word by word. (It was beyond any possibility that the subjects could have had access to the original text.) Even in those cases when the text itself was not restored to this remarkable extent, the stylistic value of the various texts was almost invariably reestablished; an originally lively, narrative story came out as a lively, narrative story , and a piece of rather dull, factual text (from a school text book on sociology) invariably came out as dull, factual prose. This experiment showed quite clearly that at least for Swedish the information contained in the combinations of surface markers to a remarkably high degree reflects the syntactic structure of the original text; in almost all cases also the stylistic value and in some few cases even the semantic content was kept. (The extent to which this is true is probably language dependent; Swedish is rather rich in morphology, and this property is certainly a contributing factor for an experiment of this type to come out successful to the extent it actually did.) This type of experiment has since then been repeated many times by many scholars; in fact, it ls one of the standard ways to demonstrate the concept of redundancy in texts. But there are several other important conclusions one could draw from this type of experiments. First of all, of course, the obvious conclusion that surface signals do carry a lot of information about the structure of sentences, probably much more than one has been inclined to think, and, consequently, It could be worth while to try to capture that Information in some kind of automatic analysis system. This is the practical side of it. But there is more to it. One must ask the question why a language llke Swedish is llke this. What are the theoretical implications? Much Interest has been devoted in later years to theories (and speculations) about human per- null ception of linguistic stimuli, and I do not think that one speculates too much if one assumes that surface markers of the type that appeared in the described experiment together constitute important clues concerning the gross syntactic structure of sentences (or utterances), clues that are probably much less consiously perceived than, e.g., the actual words in the sentences/utterances. To the extent that such clues are actually perceived they are obviously perceived simultaneously with, i.e. in parallel with, other units (words, for instance).</Paragraph>
    <Paragraph position="2"> The above way of looking upon perception as a set of independently operating processes is, of course, more or less generally accepted nowadays (cf., e.g., Lindsay-Norman 1977), and it is also generally accepted in computational linguistics that any program that aims at simulating perception in one way or other must have features that simulates (or, even better, actually performs) parallel processing, and the analysis system to be described below has much emphasis on exactly this feature.</Paragraph>
    <Paragraph position="3"> Another common saying nowadays when discussing parsing techniques is that one should try to incorporate &amp;quot;heuristic devices&amp;quot; (cf., e.g., the many subreports related to the big ARPAproject concerning Speech Recognition and Understanding 1970-76), although there does not seem to exist a very precise consensus of what exactly that would mean. (In mathematics the term has been traditionally used to refer to informal reasoning, especially when used in classroom situations. In a famous study the hungarian mathematician Polya, 1945 put forth the thesis that heuristics is one of the most important psychological driving mechanisms behind mathematical - or scientific - progress. In AIliterature it is often used to refer to shortcut search methods in semantic networks/spaces; c.f.</Paragraph>
    <Paragraph position="4"> Lenat, 1982).</Paragraph>
    <Paragraph position="5"> One reason for trying to adopt some kind of heuristic device in the analysis procedures is that one for mathematical reasons knows that ordinary, &amp;quot;careful&amp;quot;, parsing algorithms inherently seem to refuse to work in real time (i.e. in linear time), whereas human beings, on the whole, seem to be able to do exactly that (i.e. perceive sentences or utterances simultaneously with their production). Parallel processing may partly be an answer to that dilemma, but still, any process that claims to actually simulate some part of human perception must in some way or other simulate the remarkable abilities human beings have in grasping complex patterns (&amp;quot;gestalts&amp;quot;) seemingly in one single operation.</Paragraph>
    <Paragraph position="6"> Ordinary, careful, parsing algorithms are often organized according to some general principle such as &amp;quot;top-down&amp;quot;, &amp;quot;bottom-to-top&amp;quot;, &amp;quot;breadth first&amp;quot;, &amp;quot;depth first&amp;quot;, etc., these headings referring to some specified type of &amp;quot;strategy&amp;quot;. The heuristic model we are trying to work out has no such preconceived strategy built into it. Our philosophy is instead rather anarchistic (The Heuristic Principle): Whatever linguistic unit that can be identified at whatever stage of the analysis, according to whatever means there are, i_~s identified, and the significance of the fact that the unit in question has been identified is made use of in all subsequent stages of the analysis. At any time one must.be prepared to reconsider an already established analysis of a unit on the ground that evidence a~alnst the analysis may successively accumulate due to what analyses other units arrive at.</Paragraph>
    <Paragraph position="7"> In next section we give a brief description of the analysis system for Swedish that is now under development at our group in Stockholm. As has been said, much effort is spent on trying to make use of surface signals as much as possible.</Paragraph>
    <Paragraph position="8"> Not that we believe that surface signals play a more important role than any other type of linguistic signals, but rather that we think it is important to try to optimize each single sub-process (in a parallel system) as much as ~osslble, and, as said, it might be worth while to look careful into this level, because the importance of surface signals might have been underestimated in previous research. Our exneriments so far seem to indicate that they constitute excellent units to base heuristic guesses on. Another reason for concentrating our efforts on this level is that it takes time and requires much hard computational work to get such an anarchistic system to really work, and this surface level is reasonably simple to handle.</Paragraph>
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