Empirical Study of Predictive Powers of Simple Attachment 
Schemes for Post-modifier Prepositional Phrases 
Greg Whittemore Kathleen Ferrara 
Electronic Data Systems Corp. Texas A&~M University 
5951 Jefferson Street N.E. 
Albuquerque, NM 87109-3432 
Hans Brunner 
US West Advanced Technologies 
6 June 1990 
Abstract 
This empirical study attempts to find answers to 
the question of how a natural language (henceforth 
NL) system could resolve attachment of preposi- 
tional phrases (henceforth PPs) by examining nat- 
urally occurring PP attachments in typed dialogue. 
Examination includes testing predictive powers of 
existing attachment theories against the data. The 
result of this effort will be an algorithm for inter- 
preting PP attachment. 
Introduction 
Difficulty in resolving structural ambiguity in- 
volving PPs arises because of the great variety of 
syntactic structures which PPs can modify and 
the varying distances PPs may be from the con- 
stituents with which they are associated. Simple 
schemes to resolve attachments utilize information 
drawn from reported tendencies in the human pars- 
ing mechanism, such as the preference for PPs to 
attach to constituents that immediately precede 
them. It is always tempting to utilize such schemes 
in computer NL processors because they provide 
clear models for resolution that are both easy and 
cheap (in terms of steps involved) to implement. 
The problem with these schemes is that they can 
easily be made to fail by manipulating parameters 
that they 'know' nothing about, such as semantics, 
context, and intonation. Clearly, more elaborate 
schemes for attachment resolution are needed, but 
what these schemes should contain and how they 
should be implemented remain open. 
This study attempts to find answers to the ques- 
tion of how a computer program should resolve at- 
tachment by examining naturally occurring PP at- 
tachments in a typed dialogue domain drawn from 
a study by Brunner, Whittemore, Ferrara, and Hsu 
(1989). Various previously developed theories of 
PP attachment are tested against the data to see 
how well they predict correct attachments of PPs 
in the typed dialogues. The result of this effort 
will be a hypothesis of attachment resolution that 
seems to fit the data. 
Empirical overview 
The methods for generating the 13 naturally oc- 
curring dialogues are described in Brunner, et al. 
(1989). In essence, this study employed a "wiz- 
ard of Oz" paradigm in which a human confeder- 
ate -- the Wizard - simulates an advanced com- 
puter system engaged in written/interactive dia- 
logue with the experimental participant. Partici- 
pants of the study were each asked to plan a spe- 
cific travel agenda of their choice with information 
obtained solely by typing natural language mes- 
sages and requests through a VT220 terminal to 
a human-assisted travel information system located 
in a separate room. In response to this, the Wizard, 
who had access to both computerized and hard- 
copy travel data, was instructed to engage in con- 
structive and free-form dialogue with the partici- 
pant in order to best obtain the reservations and 
flight information required by them. Each dialogue 
took one and a half hours to complete, allowing 
enough time for about 70 sentences per dialogue 
23 
for a total of 910 sentences. 
In another study, Whittemore, Ferrara, and 
Brunner (1989) quantify the occurrence of PPs in 
the 13 dialogues in terms of the syntactic types to 
which they attach and the overall syntactic environ- 
ments in which they appear. Data is presented in 
terms of Tension Sites to illustrate possible syntac- 
tic attachment interpretations and actual interpre- 
tations that occurred. For instance in the sentence 
John eats his bananas in his backyard, potential at- 
tachment ambiguity lies in the fact that the PP in 
his backyard can attach to the noun phrase object 
his bananas or to the verb eats. Such positions were 
referred to as Tension Sites. All such Tension Sites 
for sentences with PPs were recorded along with 
actual attachments. Some instances were simple 
as in the example above with only a minimum of 
Tension Sites, while others were quite involved and 
had up to seven Tension Sites in which a verb and 
np-object along with the objects of five other prepo- 
sitions were available as attachment sites. Of the 
910 sentences in the 13 dialogues, 745 had instances 
of potential ambiguity in attachment. Much of the 
analysis presented in this paper is drawn from the 
Whittemore, et al. study. 
Theories of Preferenclng for Post-modifier 
PP Attachment 
Several of the PP attachment schemes available 
in the literature were used as a backdrop for ex- 
amining attachment tendencies in the typed dia- 
logues. These predictors (listed below) were basi- 
cally 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. 
The attachment predictors tested were: 
RIGHT ASSOCIATION (RA) - the tendency for 
constituents to associate with adjacent items to 
their right (Kimball 1973), also known as low at- 
tachment. Late Closure (Frazier 1979) is a similar 
notion. 
MINIMAL ATTACHMENT (MA) - the tendency to 
attach in a manner in which the least number of 
syntactic rules are employed (Frazier 1979). 
LEXICAL PREFERENCE VIA VERBS (LP) - the 
tendency for PPs to attach to verbs that have a 
preference for them (Ford, Bresnan, and Kaplan 
1982). 
LEXICAL PREFERENCE VIA NOUNS (LP) - is sim- 
ilar to verb LP, but PPs attach to nouns that may 
have a preference for them as discussed briefly ill 
Rappaport (1983). 
LEXICAL PREFERENCE VIA PREPOSITIONS (LP) 
- is similar to verb and noun LP, but prepositions 
themselves may have a tendency to seek out cer- 
tain kinds of constructions. For instance, temporal 
PPs may have a preference for attaching to enti- 
ties such as events that have temporal qualities to 
them. Prepositions acting as functors like this are 
mentioned in Wilks, Huang, and FaNs (1985). 
REFERENTIAL SUCCESS (P~S) - dictates that one 
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 Steed- 
man 1984). In a streamlined version of the theory 
(Hirst 1987), definite noun phrases require the re- 
cipient of discourse to try to make a connection 
to existing knowledge. Because of this added ef- 
fort 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. 
Success of Preferencing Schemes Against the 
Data 
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). 
RIGHT ASSOCIATION 
From the data evident in the dialogues it can be 
seen that RA seems to have a fairly strong influ- 
ence within the typed discourse domain of travel. 
As noted in the Tension Site tabulations (Whitte- 
more, et al.), low attachment was observed 55% of 
the time. However, its almost equally high failure 
24 
rate of 45% dictates that RA by itself is not a sat- 
isfactory scheme for deciding PP attachments. 
MINIMAL ATTACHMENT 
The success of MA in the attachment of PPs in 
the 13 dialogues is rather poor. Out of 488 in- 
stances 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. 
REFERENTIAL SUCCESS AND PRESUPPOSITION 
Using only definite NPs as a guide for indicat- 
ing that a noun phrase is being used to refer to 
some antecedent, strict notions of RS failed miser- 
ably -- out of 101 definite noun phrases only 12 
instances of exact match with some antecedent oc- 
curred. 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 lo- 
cated 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 exam- 
ples much machinery would be required to 'under- 
stand' when something was a 'rephrasing' or 're- 
striction' of an antecedent. 
The accompanying notion of presupposition, in 
which PP attachment to definite NPs is avoided 
when no such NP+PP already exists in the dis- 
course, would, numerically, need to be regarded 
as a semi-successful predictor of attachment site. 
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 de- 
cision 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% suc- 
cess rate. However, predicting correct attach- 
ment beyond avoiding definite NPs was not suc- 
cessfully 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 at- 
taches to the last occurring non-definite or verb in 
a RA fashion. Employing a combined presupposi- 
tion/RA approach, the success is still low -- only 
52% (or 362 attachments) are correctly predicted. 
VERB LEXICAL PREFERENCING 
To determine the success of LP of verbs in the 
13 travel dialogues, each verb used within the dia- 
logues 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 pref- 
erence for locative PPs. The rest were determined 
to be LP verbs through a consensus of 3 individu- 
als, 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 tab- 
ulated. Next, the success of the LP verbs was de- 
termined by quantifying the times that they failed 
versus the times they succeeded. Reasons for fail- 
ure in LP verbs were then sought out through all 
analysis of the sentences in which LP verbs and pos- 
sible PPs that could go with LP verbs were present, 
but the two were not associated with each other. 
A synopsis of the findings on verb LP is below. 
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 incorpora- 
tion 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 liter- 
ature for determining lexical preferencing, but it was felt at 
the time that their predictive powers were somewhat unreli- 
able, though the authors could very well be wrong. Readers 
should refer to chapter one in Somers (1987) for a good dis- 
cussion of various preference-determining schemes. 
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. 
25 
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. 
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. 
(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. 
The verb LEAVE was determined to 
have a preference for the prepo- 
sition TO, as was the verb GO. 
However, in the example TO 
attaches only to GO 
To account for the attachments some added ma- 
chinery 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 Fra- 
zier (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. 
In the travel domain in this study, with a combi- 
nation of RA and verb LP it was found that in every 
case in which 2 verbs were vying for the same PP at- 
tachment, attachment was made to the lower verb. 
With this additional machinery all but 3 of the in- 
correct attachments in sentences with LP verbs can 
be explained. 
In the 3 remaining instances in which attach- 
ment 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 at- 
tachment 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. 
(2). I need to know would you like for 
me to SHOW you some FLIGHT 
schedules to Dublin? 
NOUN LP FOR PPS 
The methodology for exploring noun LP was sim- 
ilar 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 at- 
tachments 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. 
(3). Would you like for me to show you 
some FLIGHTS TO Dublin? 
(4). What is the round trip FAKE for 
Aer Lingus and for British 
Airlines FROM JFK on August 30 
TO Dublin returning Sept 217 
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 predic- 
tions 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" 
to the domain at hand. 
26 
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 oc- 
curring 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 sepa- 
rate FLIGHT from London to Dublin. 
Similarly, when deriving interpretations in which 
LP verbs are followed by LP nouns, RA between the 
competing LP words makes the correct interpreta- 
tion. Thus in the 3 sentences in which LP verbs are 
followed by LP nouns, and LP verbs and nouns pre- 
fer 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. 
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. 
MODIFYIN~ PPS (OR 1"1" L1") 
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. 
Since for the remaining PPs, the predominant 
general preference schemes were either not appro- 
priate (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 func- 
tions 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, tem- 
poral 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). 
(6). TEMPORAL. British Airlines has a 
flight that leaves AT 12:30. 
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. 
For the PPs involved in LP, it could be argued 
that their attachment is determined by the near ne- 
cessity 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. 
For temporals, attachment sites are either ac- 
tions 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 temporal- 
accepting noun or verb and RA proved to be suc- 
cessful. 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 attach- 
ment of temporal-PPs was successfully predicted in 
all but one of the 81 instances. 
For locative-PP modifiers, using the same scheme 
as was used for temporal-PP modifiers in which af- 
ter noun and verb LP fail a search is performed 
for the last locative-accepting item to the left, pre- 
dictability of attachment of locative-PPs was again 
almost 100%. 4 
The resulting preferencing scheme for temporal- 
locative-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 re- 
quire 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). 
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. 
27 
left that has a head with a locative 
quality. 
- If there is a temporal PP, attach to 
the most adjacent constituent to 
the left that has a head with a 
temporal quality. 
added notes: 
Must be able to link up with 
EXTRACTED elements. 
Characteristics of EXTRACTED elements 
must be ~ssociated with their gaps 
before linking locative PPs is 
attempted. 
Must first link any temporal/locative 
qualities of modifying adjectives to 
the modified head. 
OTHER PP MODIFIERS 
The remaining PP modifiers, those that are prob- 
ably not sought after by an LP verb or noun and do 
not belong to the class of temporal-PPs or locative- 
PPs, were treated together. The reason for this 
particular grouping was that there were a num- 
ber of functions evident in some PPs that occurred 
very infrequently and since one of the major foci 
of the study was to try to find general means of 
deciding attachment of PPs, individualization of 
these PPs was, at first, discounted. In some of 
the prior attachment schemes, there were some el- 
ements that were given the power to seek out some 
other constituent (e.g. LP verb sought out cer- 
tain case types presented in particular PPs and 
temporal PPs sought out temporal-bearing nouns 
or verbs). Attempting to use LP with the varied 
other group was not possible since no one function 
type (e.g. such as temporality) and no single pref- 
erence characteristic was evident. Other schemes 
were necessary for this group. 
What proved to be succesful was the Hirst (1987) 
modified version of presupposition in which attach- 
ment to definites is generally avoided. Adding the 
notion of RA, one can also decide between equally 
weighted non-definites and verbs when both are 
present. 
The combined presupposition-RA algorithm is 
expressed below. When coming upon a PP that 
was of the other type, an attachment is made to 
the most recent verb or non-definite noun in a RA 
fashion. 
Avoid attachment to definite NPs and 
attach to most recently occurring 
verb or non-definite NP to the left. 
As shown below under this scheme, correct pre- 
diction was made 100% of the time for the non- 
definite+verb grouping. However, when examining 
the success of attachment with the definite NPs, 
the rate of successful prediction was much lower. 
In 13 instances, avoiding attachment to definite 
NPs was the correct thing to do, but 7 times it 
was not, resulting in a 65% success rate. Thus if 
one permits the RA+non-definite noun preferenc- 
ing scheme, the only items needing further expla- 
nation are the definite NPs. 
of correct predictions of attaching 
"other" PPs to last occurring avail- 
able verb or non-definite noun to 
the left I00~ 
of correct prediction to avoid 
attachment to definite NPs. 65X 
With the limited group of 7 definite NPs (these 
were the remaining, unresolved definite NPs), it 
was easy to identify a single class to which the con- 
flicting NPs belonged. All the nouns but one 5 that 
could be associated with PPs were ones that could 
be used in partitive expressions. Partitive nouns 
can be separated out from other nouns as those 
noun expressions that denote a kind or quantity 
and are typically followed by the preposition of. In 
(6) are two examples from the dialogues. 
(6) a. the legs of your trip. 
b. the size of the hotel 
The algorithm for the other group is: 
Check to see if preceding lowest consti- 
tuent is a definite NP and part of a 
partitive expression, 
If it is, attach the PP to the preceding 
definite NP, 
Otherwise, attach to the most recently 
occurring verb or non-definite NP. 
5The sole exception was with the noun \]eeling in the ex- 
pression the \]eeling o\] the community. It is highly probable 
that this is an idiomatic noun phrase and should be entered 
in an idiomatic lexicon. 
28 
Overall Algorithm 
As laid out below after some preliminary tasks 
are performed, namely associating nouns with their 
adjectives and extracted items with their gaps, the 
first preference to apply is noun and verb LP. If 
noun and verb LP fails, the two-stepped tempo- 
ral/locative modifer preference can step in and per- 
form attachments of which it is capable. When all 
else fails, the other modifier routine finishes off 
anything left over. 
Associate adjectives with locative (and 
possibly temporal) qualities to the 
nouns they modify. 
Associate extracted items with their 
respective 'gaps.' 
If an LP verb or LP noun is present, 
apply verb or noun LP. 
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. 
Otherwise, if a temporal PP is present, 
attach it to the most adjacent consti- 
tuent to the left whose head contains 
a temporal quality. 
Otherwise, if a locative PP is present, 
attach it to the most adjacent consti- 
tuent to the left whose head contains 
a locative quality. 
Otherwise, if an OTHER modifier (not a 
temporal or a locative) is present 
and if the immediately preceding 
element is a definite NP that could 
be part of a partitive expression, 
then attach the PP to the NP, 
Otherwise attach to the last occurring 
verb or non-definite NP. 
Conclusion 
The study indicates that there seems to be a 
way of predicting PP attachment in the typed in- 
teractive mode of communication by fairly sim- 
ple means. By using LP for nouns, verbs and 
prepositions (temporal and locative PPs seek out 
temporal- or locative-accepting elements) and a 
variation on the Crain and Steedman notion of 
presupposition, attachments are essentially always 
predictable. 
Correct interpretation of the 724 instances it~ 
which there existed structural ambiguity in the at- 
tachment of PPs to nouns or verbs occurred as fol- 
lows: 
Verb LP 228 instances 
Noun LP 183 instances 
Temporal prep. LP 189 instances 
Locative prep. LP 90 instances 
Other modifiers 34 instances 
(presupposition 
+ RA) 
:added note - two items were not 
accounted for: 
--- one seemed to be an idiomatic 
expression 
--- one may possibly have been 
contextually related 
RA played a role within each preferencing scheme 
as did a weak notion of plausibility. RA was used 
as the arbitrator whenever there remained an intra- 
conflict in a preferencing algorithm (and sometimes 
when there was inter-conflict between schemes). 
The use of plausibility to talk about relationships 
between verbs or nouns and associated PPs was 
thought to be a necessary notion in that simple 
searches for only prepositions were deemed to be 
too weak of a notion. When verb or noun LP was 
at work, nouns and verbs sought out PPs (as op- 
posed to single prepositions) that as a whole had 
some attribute(s) necessary to fulfill some semantic 
requirements. Sometimes PPs also had to be con- 
cluded to be of a particular type in order to search 
out a unique kind of noun or verb. Apparently, PP 
Lexical Preferencing allowed PPs that were tempo- 
ral or locative in nature to look for nouns and verbs 
that bore temporal or locative characteristics, re- 
spectively. Referential Success in its pure sense was 
a poor predictor of attachments. However, the re- 
lated notions of presupposition regarding definites, 
indefinites, etc. were good predictors of attachment 
for a small number of PPs. 
Finally, a more cognitive finding resulting from 
the study was the great predictability of attach- 
ment, suggesting that there is something about the 
typed interactive mode of communication that coil- 
strains the possibilities on attachment such that 
attachment always goes with the unmarked ce, sc. 
There are at least three pressures that may help 
to make these constraints come about. One is the 
29 
lack of the spoken element which carries with it 
intonation patterns and variations in pausing that 
can influence the ways that one parses. One must 
rely on only the cues available by written means 
to aid in disambiguating attachments. Secondly, 
the added comparative slowness at which interlocu- 
tors type and the resulting tendency to leave out 
unnecessary punctuation marks often useful in dis- 
ambiguating text makes yet a further constrained 
subset. Thirdly, a speaker may be aware of the 
time lag (hence taxation on memory) that exists 
between typing some modified element and its as- 
sociated PP. The lag may have an effect on how 
such pairs are presented. Prominent ways of high- 
lighting the links may depend more on notions such 
as LP or RA that might not be needed as much in 
other modes of communication. These factors to- 
gether may make it necessary for participants in 
typed interactive communication to rely on a set of 
default structures that each can cue on easily. 
A cknowledgements 
We wish to thank Joyce Conner for her time 
and energy spent in collecting and analyzing the 
data, Melissa Macpherson for her insights into the 
notions presented in the paper, and Laurie Whit- 
temore and Jim Barnett for their editing efforts. 
Also, much of the work on this paper was car- 
ried out when Greg Whittemore and Kathy Ferrara 
were employees of MCC, and thanks goes to MCC 
personnel, particularly Elaine Rich, who made it 
possible for the study to be performed. 
References 
\[1\] Allerton, D. 1982. VALENCY AND THE EN- 
GLISH VERB. London: Academic Press. 
\[2\] Brunner, H., Whittemore, G., Ferrara, K., Hsu, 
J., 1988. An assessment of written/interactive 
dialogue for information retrieval applications. 
(MCC Technical Report #ACT-HI-245-89). 
\[3\] Crain, S. and Steedman, M. 1984. On not be- 
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