<?xml version="1.0" standalone="yes"?>
<Paper uid="C96-2123">
  <Title>On the Structural Complexity of Natural Language Sentences</Title>
  <Section position="2" start_page="0" end_page="729" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Intuitive\]y, certain syntactic structures arc uLore difficult for htnnans to process thau others. For example, compare the following to sentences: (1) a. 'Fhe cat that the dog that the man bought chased died.</Paragraph>
    <Paragraph position="1"> b. The man bought the dog that chased the cat that died.</Paragraph>
    <Paragraph position="2"> It is ohvious that sentence (la.) is much mor(' difficult to understarld than (1 b). Since the two sentences are of the same length an(l involve the same set of semantic relationships, the ditliculty in rmderstan(ling (1 a) can only be attributed to its syntactic structure.</Paragraph>
    <Paragraph position="3"> '\['he objecl;ive of this pal)er is to fortnalize the intuition a.bout the complexity of syutactic stru(&gt; tures. We propose a detinition of structural colnI)h~xil;y (SC) such thai; sentences ranked by our definition as more complex are generally more difficult for humans to process than otherwise similar sentences, hi other words, suppose a pair of sentences A arid B consist of the same set of words and have essentially the same meaning, then sentence A is more difficult to process than sentence 1~ if SC(A)&gt;SC(B). For example, the proposed detinition of structural complexity correctly pre*On lea,re Dora the University of Manitoba., Winnipeg, M~mitoba, (\]~tnmla. This rt,.se,Lrch has 1)een supported by NSli',II.(\] ltcsearch (',rant OG1)121338. The author is very gr~teful to the reviewers who pointed oat several mistakes in the draft.</Paragraph>
    <Paragraph position="4"> dicts that (la) ix much more difficult to process than (lb).</Paragraph>
    <Paragraph position="5"> 'I'll(: notiou of structural complexity proposed in this l)apc'r oilers explanations \['or a set of seemiugly unrelated phenomena: * We will show dlat the definition of structural comph:xity explains why a I)utch sentence involving cross-serial dependencies is sliglrdy easier to underst~md than a corresponding cenl, er-embedded German sentence.</Paragraph>
    <Paragraph position="6"> * We will also show that extrapositions, such as heavy-NP shift and PP extractions are motivated by reducing syntactic complexity. The extraposition of an element is only warranted when the. structural COml)lexity of the sen-I.en(:e is reduced as a result.</Paragraph>
    <Paragraph position="7"> * NP ntodifiers of a head tend to be closer to the head than its PP modifiers, which in turn tend to be closer than its CP (clausal) moditiers. In Generalized Phrase Strcuture Grammar ((~VS(~) (Gazd~u&amp;quot; ctal., 1985), these linear order constraints are stated explicitly in the gralIllnar. The notion of structured complexity provides an explanatory account.</Paragraph>
    <Paragraph position="8"> 'l'here are several reasons why the notion of strHCtltl:a\] COml)lexity ix tlseful. Firstly, in natural language generation, a generator should get&gt; era.re the simphest sentence that conveys the intended meanings. Structural complexity can be used to choose l;he syntactic strnctures with l;he lowest structural complexity so that the resulting sentence is easier to understand than other alternatives. null Secondly, structural complexity is also needed in assessing the readability of dommtents. \[t is well known that the length of a sentence is not, a relit~ble indicator of its readability. Yet, the readability of texts has up to now heen measured by tlJe lengths of sentences and familiarities of th(: words in the documents. Using structural complexity instead of sentence length allows the read~fl)ility of documents to be measured tnorc accurately. null Finally, we propose, in Section 4, that extrapositions ~re rnotiw~ted by reduction of structural  complexity. In other words, extrapositions are only allowed if the structural complexity of tile sentence is reduced as a result. This constraint is nsefnl both in parsing sentences with extrapositions and in deciding where to use extraposition during generation.</Paragraph>
    <Paragraph position="9"> The notion of structural complexity is defined in Section 2. We then justify the definition of structural complexity by demonstrating in Sections 3, 4, and 5 that sentences with lower structural complexity are easier to understand than otherwise similar sentences with higher structural complexity. null</Paragraph>
  </Section>
class="xml-element"></Paper>