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<Paper uid="P90-1004">
  <Title>Empirical Study of Predictive Powers of Simple Attachment Schemes for Post-modifier Prepositional Phrases</Title>
  <Section position="2" start_page="23" end_page="23" type="metho">
    <SectionTitle>
PP Attachment
</SectionTitle>
    <Paragraph position="0"> Several of the PP attachment schemes available in the literature were used as a backdrop for examining attachment tendencies in the typed dialogues. These predictors (listed below) were basically employed as individual templates which were applied against the data. Percentages of correct predictability were recorded and some investigation into their failures was made. Only attachments to nouns and verbs were made in this study, giving a corpus of 724 examples.</Paragraph>
    <Paragraph position="1"> The attachment predictors tested were:</Paragraph>
  </Section>
  <Section position="3" start_page="23" end_page="23" type="metho">
    <SectionTitle>
RIGHT ASSOCIATION (RA) - the tendency for
</SectionTitle>
    <Paragraph position="0"> constituents to associate with adjacent items to their right (Kimball 1973), also known as low attachment. Late Closure (Frazier 1979) is a similar notion.</Paragraph>
    <Paragraph position="1"> MINIMAL ATTACHMENT (MA) - the tendency to attach in a manner in which the least number of syntactic rules are employed (Frazier 1979).</Paragraph>
  </Section>
  <Section position="4" start_page="23" end_page="23" type="metho">
    <SectionTitle>
LEXICAL PREFERENCE VIA VERBS (LP) - the
</SectionTitle>
    <Paragraph position="0"> tendency for PPs to attach to verbs that have a preference for them (Ford, Bresnan, and Kaplan 1982).</Paragraph>
  </Section>
  <Section position="5" start_page="23" end_page="23" type="metho">
    <SectionTitle>
LEXICAL PREFERENCE VIA NOUNS (LP) - is sim-
</SectionTitle>
    <Paragraph position="0"> ilar to verb LP, but PPs attach to nouns that may have a preference for them as discussed briefly ill Rappaport (1983).</Paragraph>
  </Section>
  <Section position="6" start_page="23" end_page="23" type="metho">
    <SectionTitle>
LEXICAL PREFERENCE VIA PREPOSITIONS (LP)
</SectionTitle>
    <Paragraph position="0"> - is similar to verb and noun LP, but prepositions themselves may have a tendency to seek out certain kinds of constructions. For instance, temporal PPs may have a preference for attaching to entities such as events that have temporal qualities to them. Prepositions acting as functors like this are mentioned in Wilks, Huang, and FaNs (1985).</Paragraph>
  </Section>
  <Section position="7" start_page="23" end_page="23" type="metho">
    <SectionTitle>
REFERENTIAL SUCCESS (P~S) - dictates that one
</SectionTitle>
    <Paragraph position="0"> first checks to see if there are any 'like' entities ill the discourse, namely ones that have similar PPs as modifiers. If there are matches, then attachment takes on the same look as the antecedent. There are also notions of presupposition in the theory that make predictions about definite, indefinite, generic, and generic plural noun phrases (Crain and Steedman 1984). In a streamlined version of the theory (Hirst 1987), definite noun phrases require the recipient of discourse to try to make a connection to existing knowledge. Because of this added effort in which one must search his discourse space, it has been predicted that attachment to a definite noun phrase would be less preferred. Other noun phrases -- indefinites, generics, and bare plurals -- along with verbs are preferred over definites as attachment sites since they supposedly require less search over discourse space.</Paragraph>
    <Section position="1" start_page="23" end_page="23" type="sub_section">
      <SectionTitle>
Success of Preferencing Schemes Against the
Data
</SectionTitle>
      <Paragraph position="0"> The 'effect' that each of the preferencing schemes reviewed above has on the attachment of the post-modifiers is explored in the remaining sections. Not every possible PP attachment found in the corpus is examined. An attempt is made to explain only attachments to nouns and verbs (thus those made to adverbs, adjectives, prepositions themselves, or within idiomatic expressions are excluded).</Paragraph>
    </Section>
  </Section>
  <Section position="8" start_page="23" end_page="24" type="metho">
    <SectionTitle>
RIGHT ASSOCIATION
</SectionTitle>
    <Paragraph position="0"> From the data evident in the dialogues it can be seen that RA seems to have a fairly strong influence within the typed discourse domain of travel.</Paragraph>
    <Paragraph position="1"> As noted in the Tension Site tabulations (Whittemore, et al.), low attachment was observed 55% of the time. However, its almost equally high failure  rate of 45% dictates that RA by itself is not a satisfactory scheme for deciding PP attachments.</Paragraph>
  </Section>
  <Section position="9" start_page="24" end_page="24" type="metho">
    <SectionTitle>
MINIMAL ATTACHMENT
</SectionTitle>
    <Paragraph position="0"> The success of MA in the attachment of PPs in the 13 dialogues is rather poor. Out of 488 instances in which there was an opportunity for MA to take a role, only 177 examples (or 36%) behaved according to a strict notion of MA. By a strict notion we mean that whenever possible, the least number of rules are applied.</Paragraph>
  </Section>
  <Section position="10" start_page="24" end_page="24" type="metho">
    <SectionTitle>
REFERENTIAL SUCCESS AND PRESUPPOSITION
</SectionTitle>
    <Paragraph position="0"> Using only definite NPs as a guide for indicating that a noun phrase is being used to refer to some antecedent, strict notions of RS failed miserably -- out of 101 definite noun phrases only 12 instances of exact match with some antecedent occurred. There were also 17 cases in which some subsequent phrase was used to 'restrict' or refer to some semantic subset of an antecedent. There was one additional case in which a subsequent noun phrase was a rephrasing of an antecedent. For the remaining 71 instances, no antecedent could be located within the text. Altogether there were only 30 out of 101 that could be deemed successful. It should also be noted that for a NL understanding system to correctly interpret just these few examples much machinery would be required to 'understand' when something was a 'rephrasing' or 'restriction' of an antecedent.</Paragraph>
    <Paragraph position="1"> The accompanying notion of presupposition, in which PP attachment to definite NPs is avoided when no such NP+PP already exists in the discourse, would, numerically, need to be regarded as a semi-successful predictor of attachment site.</Paragraph>
    <Paragraph position="2"> Disregarding the 30 cases in which an antecedent for an NP was found in the discourses, one would have to say that avoiding attachment to NP was successful since for the remaining 694 instances (724 total minus the 30 cases above) correct decision attachment was made to avoid attachment to definite NPs 623 times (694 cases minus the 71 cases of non-anaphoric NP+PPs) for a 90% success rate. However, predicting correct attachment beyond avoiding definite NPs was not successfully performed. It is not enough to just try to avoid attaching to definite NPs; there must also be a way of specifying how PPs are to link up with other non-definites and verbs. In the study, Hirst's (1987) modified version was used in which one attaches to the last occurring non-definite or verb in a RA fashion. Employing a combined presupposition/RA approach, the success is still low -- only 52% (or 362 attachments) are correctly predicted.</Paragraph>
  </Section>
  <Section position="11" start_page="24" end_page="25" type="metho">
    <SectionTitle>
VERB LEXICAL PREFERENCING
</SectionTitle>
    <Paragraph position="0"> To determine the success of LP of verbs in the 13 travel dialogues, each verb used within the dialogues was examined for its potential for LP. Some verbs were determined to have a very strong LP such as some two part verbs like involved in or verbs like live that have an obviously strong preference for locative PPs. The rest were determined to be LP verbs through a consensus of 3 individuals, and when possible, further substantiated to be LP verbs through the aid of two sources on verbs and their complements - A COMPLETE GRAM-MAR OF ENGLISH by Quirk, Greenbaum, Leech, and Svartvik (1972) and VALENCY OF VERBS by Allerton (1982). 1 After a complete list of the verbs was derived, the number of times that the verbs appeared with sought-after prepositions was determined and tabulated. Next, the success of the LP verbs was determined by quantifying the times that they failed versus the times they succeeded. Reasons for failure in LP verbs were then sought out through all analysis of the sentences in which LP verbs and possible PPs that could go with LP verbs were present, but the two were not associated with each other.</Paragraph>
    <Paragraph position="1"> A synopsis of the findings on verb LP is below.</Paragraph>
    <Paragraph position="2"> The main point to be gleaned from this synopsis is that there seem to be a fairly large number of PP attachments that could be construed to be the result of verb LP -- 228 out of 724 total. This is significant because it indicates that the incorporation of an accurate LP scheme could be beneficial in a PP attachment resolution scheme. 2 verb lexical preferencing: 228 instances of verb LP 1There have been several methods suggested in the literature for determining lexical preferencing, but it was felt at the time that their predictive powers were somewhat unreliable, though the authors could very well be wrong. Readers should refer to chapter one in Somers (1987) for a good discussion of various preference-determining schemes.</Paragraph>
    <Paragraph position="3"> 2Closer scrutiny of the different LP verbs also made it apparent that the number of domain-specific LP verbs is comparatively quite large. For instance, the verbs begin, book, change, depart, fly, get, and leave, to name some, all have senses that seemed particular to the travel domain.  47 different verbs examples: arranged through, arrive at, begin from, fly from/to, start at The tabulations shown above are only for correct attachments in which it could be decided that a particular LP verb did attach to a PP. There were also 21 LP verbs that failed to link up with existing PPs that they normally seek.</Paragraph>
    <Paragraph position="4"> Verb-LP alone failed in 18 of the 21 instances, seemingly because of the presence of multiple LP verbs. In (1) is an example from the dialogues.</Paragraph>
    <Paragraph position="5"> (I). Before deciding that I want to know the flight times for United Air Lines LEAVING from Austin and GOING TO JFK in New York on August 30.</Paragraph>
    <Paragraph position="6"> The verb LEAVE was determined to have a preference for the preposition TO, as was the verb GO.</Paragraph>
    <Paragraph position="7"> However, in the example TO attaches only to GO To account for the attachments some added machinery is needed. It was earlier demonstrated that there was a 54% tendency for attachment of PPs to be to the most immediate low constituent to their left, or Right Association - RA. RA has also been shown in the work of Wilks et al. (1985) and Frazier (1979) to be beneficial when choosing between two LP verbs. They predict that when multiple LP verbs appear a sought after PP attaches to the last LP verb that precedes it.</Paragraph>
    <Paragraph position="8"> In the travel domain in this study, with a combination of RA and verb LP it was found that in every case in which 2 verbs were vying for the same PP attachment, attachment was made to the lower verb. With this additional machinery all but 3 of the incorrect attachments in sentences with LP verbs can be explained.</Paragraph>
    <Paragraph position="9"> In the 3 remaining instances in which attachment goes against the notion of LP, attachments were made to nouns. In (2) is one of the instances. In (2), show was deemed as normally calling for a PP headed by lo, but attachment went to the NP object following the verb. Under a strict notion of verb LP there is no provision to allow the attachment of PPs to nouns following LP verbs. The possibility of nouns having LP characteristics will be explored in the next section, and the example below should be re-examined in light of the data there.</Paragraph>
    <Paragraph position="10"> (2). I need to know would you like for me to SHOW you some FLIGHT schedules to Dublin?</Paragraph>
  </Section>
  <Section position="12" start_page="25" end_page="27" type="metho">
    <SectionTitle>
NOUN LP FOR PPS
</SectionTitle>
    <Paragraph position="0"> The methodology for exploring noun LP was similar to that of verb LP. Shown below are the overall results for noun LP. As indicated, the number of PPs attaching to LP nouns is again comparatively quite large, almost as large as the number of attachments to LP verbs -- 183 versus 228. Thus, as is the case for LP verbs, noun LP seems to be a significant means by which PP attachments can be predicted. 3 noun lexical preferencing 183 instances of noun LP 24 different ip nouns examples: (air)fare(s) from/to, bus to, carrier from/to, and travel(ing) by, Under the LP noun analysis, all instances in which there was a single LP noun were correctly accounted for by a noun LP scheme. Under a LP noun analysis PPs that were at a proximal, such as (3), or great distance, such as (4), were able to  correctly link up with appropriate nouns.</Paragraph>
    <Paragraph position="1"> (3). Would you like for me to show you some FLIGHTS TO Dublin? (4). What is the round trip FAKE for  There were three sentences in which multiple LP words appeared in which there was first an LP noun, and later either another LP noun or an LP verb. With these, using the same RA analysis that was employed for LP words, correct predictions about attachment can be made - when any 3Again, as with the LP verbs, there are many nouns that seem to have LP for the travel domain. The nouns bus, carrier, ehan#e, connectians, dollars, airfare, flights, one way, travel, and roundtrip all seem to have senses particulaa&amp;quot; to the domain at hand.</Paragraph>
    <Paragraph position="2">  two LP words that seek the same PP are present, no matter if they are nouns or verbs, attachment is made to the latter LP word. For instance, sentence (5) has two LP nouns, tr/p and flight, both of which were deemed to have a preference for the singly occurring PP headed by from. By enforcing RA, in which the attachment of the from PP is made to the last occurring and lowest LP noun (in this case flight), the correct interpretation can be derived. (5). Then what you would rather have is a round TRIP to London) with a separate FLIGHT from London to Dublin.</Paragraph>
    <Paragraph position="3"> Similarly, when deriving interpretations in which LP verbs are followed by LP nouns, RA between the competing LP words makes the correct interpretation. Thus in the 3 sentences in which LP verbs are followed by LP nouns, and LP verbs and nouns prefer the same PPs, RA attachment is favored with attachment to the three last occurring LP nouns. The combined noun and verb LP scheme is: If an LP verb or LP noun is present, apply verb or noun LP.</Paragraph>
    <Paragraph position="4"> If two LP verbs or nouns are present that seek the same PP use the notion of RA and attach the PP to the last word that seeks it.</Paragraph>
    <Paragraph position="5"> MODIFYIN~ PPS (OR 1&amp;quot;1&amp;quot; L1&amp;quot;) The verb and noun LP schemes demonstrated above were successful but only for the cases in which LP verbs and nouns appeared. Excluding the 411 PPs that seemed to be accounted for via LP, there still remain to be explained 313 PPs, 43% of the cases.</Paragraph>
    <Paragraph position="6"> Since for the remaining PPs, the predominant general preference schemes were either not appropriate (verb LP, noun LP, or RS) or shown not to be powerful enough predictors by themselves (RA and MA), the PPs were examined in terms of the functions they served in hopes that some generalities amongst them would become evident. This proved to be a promising exercise since most of the PPs were found to belong to two function types, temporal and locative indicators. Of the remaining PPs, 189 (60% of the remaining) were temporal, 90 (28%) were locative, and 34 (12%) were of a mixed variety. Some examples of these are provided in (0).</Paragraph>
    <Paragraph position="7"> (6). TEMPORAL. British Airlines has a flight that leaves AT 12:30.</Paragraph>
    <Paragraph position="8"> LOCATIVE. Could you suggest a few hotels in a moderate price range IN a nice part of London? OTHER/MIXED. Please book me on these flights WITH an aisle seat.</Paragraph>
    <Paragraph position="9"> For the PPs involved in LP, it could be argued that their attachment is determined by the near necessity that some argument position for a LP head be filled. With the remaining PPs, there seemed to be something else required in order to make their attachment. Instead of having something look for the PPs, it appeared that there needed to be a way by which the PPs could serve as functors in which they seek out arguments (a notion also defended ill Bresnan, 1982). The items to which the temporal and locative PPs attach are ones that have some temporal or locative quality to them.</Paragraph>
    <Paragraph position="10"> For temporals, attachment sites are either actions that can occur at some particular time or some state that must last for some period of time. In the type-written dialogues in the travel domain, the combination of leftward search for a temporalaccepting noun or verb and RA proved to be successful. With a combined PP LP/RA algorithm in which temporal-PPs look for the first NP or VP to their left that has a temporal quality, the attachment of temporal-PPs was successfully predicted in all but one of the 81 instances.</Paragraph>
    <Paragraph position="11"> For locative-PP modifiers, using the same scheme as was used for temporal-PP modifiers in which after noun and verb LP fail a search is performed for the last locative-accepting item to the left, predictability of attachment of locative-PPs was again almost 100%. 4 The resulting preferencing scheme for temporallocative-PP LP is: - MUST be ordered after noun and verb LP - If there is a locative PP, attach to the most adjacent constituent to the 4Actually, out of the 90 instances of locative PPs (this excludes those PPs that are called for by LP words) 8 require further elaboration. Examples of further elaboration are permitting gapping out of complex NPs so that PPs can attach to their 'extracted' elements as in (a) and having mechanisms to derive compound nouns and adjective/noun combinations as in (b).</Paragraph>
    <Paragraph position="12">  a. Which airport do you want to fly to *GAP* in Paris? b. Provide DEPARTURE TIMES fi'om Dublin o,~ 9/20/86 to Boston with ARRIVAL TIMES in Boston.  left that has a head with a locative quality.</Paragraph>
    <Paragraph position="13"> - If there is a temporal PP, attach to the most adjacent constituent to the left that has a head with a temporal quality.</Paragraph>
    <Paragraph position="14"> added notes: Must be able to link up with EXTRACTED elements.</Paragraph>
    <Paragraph position="15"> Characteristics of EXTRACTED elements must be ~ssociated with their gaps before linking locative PPs is attempted.</Paragraph>
    <Paragraph position="16"> Must first link any temporal/locative qualities of modifying adjectives to the modified head.</Paragraph>
  </Section>
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