An Unsupervised Approach to Prepositional Phrase Attachment
using Contextually Similar Words
Patrick Pantel and Dekang Lin
Department of Computing Science
University of Alberta 1
Edmonton, Alberta T6G 2H1 Canada
{ppantel, lindek}@cs.ualberta.ca
                                                  
1 This research was conducted at the University of Manitoba.
Abstract
Prepositional phrase attachment is a
common source of ambiguity in natural
 processing. We present an
unsupervised corpus-based approach to
prepositional phrase attachment that
achieves similar performance to supervised
methods. Unlike previous unsupervised
approaches in which training data is
obtained by heuristic extraction of
unambiguous examples from a corpus, we
use an iterative process to extract training
data from an automatically parsed corpus.
Attachment decisions are made using a
linear combination of features and low
frequency events are approximated using
contextually similar words.
Introduction
Prepositional phrase attachment is a common
source of ambiguity in natural 
processing. The goal is to determine the
attachment site of a prepositional phrase in a
sentence. Consider the following examples:
1.  Mary ate the salad with a fork.
2.  Mary ate the salad with croutons.
In both cases, the task is to decide whether the
prepositional phrase headed by the preposition
with  attaches to the noun phrase (NP) headed by
salad  or the verb phrase (VP) headed by ate . In
the first sentence, with  attaches to the VP since
Mary is using a fork to eat her salad. In sentence
2, with  attaches to the NP since it is the salad
that contains croutons.
Formally, prepositional phrase attachment is
simplified to the following classification task.
Given a 4-tuple of the form ( V , N 1 , P , N 2 ), where
V  is the head verb, N 1 is the head noun of the
object of V , P  is a preposition, and N 2  is the head
noun of the prepositional complement, the goal
is to classify as either adverbial attachment
(attaching to V) or adjectival attachment
(attaching to N 1 ). For example, the 4-tuple ( eat ,
salad , with , fork ) has target classification V .
In this paper, we present an unsupervised
corpus-based approach to prepositional phrase
attachment that outperforms previous
unsupervised techniques and approaches the
performance of supervised methods. Unlike
previous unsupervised approaches in which
training data is obtained by heuristic extraction
of unambiguous examples from a corpus, we use
an iterative process to extract training data from
an automatically parsed corpus. The attachment
decision for a 4-tuple ( V , N 1 , P , N 2 ) is made as
follows. First, we replace V  and N 2  by their
contextually similar words and compute the
average adverbial attachment score. Similarly,
the average adjectival attachment score is
computed by replacing N 1 and N 2 by their
contextually similar words. Attachment scores
are obtained using a linear combination of
features of the 4-tuple. Finally, we combine the
average attachment scores with the attachment
score of N 2 attaching to the original V  and the
attachment score of N 2  attaching to the original
N 1. The proposed classification represents the
attachment site that scored highest.
1 Previous Work
Altmann and Steedman (1988) showed that
current discourse context is often required for
disambiguating attachments. Recent work shows
that it is generally sufficient to utilize lexical
information (Brill and Resnik, 1994; Collins and
Brooks, 1995; Hindle and Rooth, 1993;
Ratnaparkhi et al., 1994).
One of the earliest corpus-based approaches to
prepositional phrase attachment used lexical
preference by computing co-occurrence
frequencies (lexical associations) of verbs and
nouns with prepositions (Hindle and Rooth,
1993). Training data was obtained by extracting
all phrases of the form ( V , N 1 , P , N 2 ) from a
large parsed corpus.
Supervised methods later improved
attachment accuracy. Ratnaparkhi et al. (1994)
used a maximum entropy model considering
only lexical information from within the verb
phrase (ignoring N 2). They experimented with
both word features and word class features, their
combination yielding 81.6% attachment
accuracy.
Later, Collins and Brooks (1995) achieved
84.5% accuracy by employing a backed-off
model to smooth for unseen events. They
discovered that P  is the most informative lexical
item for attachment disambiguation and keeping
low frequency events increases performance.
A non-statistical supervised approach by Brill
and Resnik (1994) yielded 81.8% accuracy using
a transformation-based approach (Brill, 1995)
and incorporating word-class information. They
report that the top 20 transformations learned
involved specific prepositions supporting
Collins and Brooks’ claim that the preposition is
the most important lexical item for resolving the
attachment ambiguity.
The state of the art is a supervised algorithm
that employs a semantically tagged corpus
(Stetina and Nagao, 1997). Each word in a
labelled corpus is sense-tagged using an
unsupervised word-sense disambiguation
algorithm with WordNet (Miller, 1990). Testing
examples are classified using a decision tree
induced from the training examples. They report
88.1% attachment accuracy approaching the
human accuracy of 88.2% (Ratnaparkhi et al.,
1994).
The current unsupervised state of the art
achieves 81.9% attachment accuracy
(Ratnaparkhi, 1998). Using an extraction
heuristic, unambiguous prepositional phrase
attachments of the form ( V , P , N 2 ) and ( N 1 , P ,
N 2) are extracted from a large corpus. Co-
occurrence frequencies are then used to
disambiguate examples with ambiguous
attachments.
2 Resources
The input to our algorithm includes a collocation
database and a corpus-based thesaurus, both
available on the Internet 2. Below, we briefly
describe these resources.
2.1 Collocation database
Given a word w  in a dependency relationship
(such as subject  or object ), the collocation
database is used to retrieve the words that
occurred in that relationship with w , in a large
corpus, along with their frequencies (Lin,
1998a). Figure 1 shows excerpts of the entries in
                                                  
2 Available at www.cs.ualberta.ca/~lindek/demos.htm.
eat :
object: almond 1, a pple 25, bean 5, beam 1, binge 1,
bread 13, cake 17, cheese 8, dish 14,
disorder 20, egg 31, grape 12, grub 2, hay 3,
junk 1, meat 70, poultry 3, rabbit 4, soup 5,
sandwich 18, pasta 7, vegetable 35, ...
subject: adult 3, animal 8, beetle 1, cat 3, child 41,
decrease 1, dog 24, family 29, guest 7, kid
22, patient 7, refugee 2, rider 1, Russian 1,
shark 2, something 19, We 239, wolf 5, ...
salad :
adj-modifier: assorted 1, crisp 4, fresh 13, good 3, grilled
5, leftover 3, mixed 4, olive 3, prepared 3,
side 4, small 6, special 5, vegetable 3, ...
object-of: add 3, consume 1, dress 1, grow 1, harvest 2,
have 20, like 5, love 1, mix 1, pick 1, place
3, prepare 4, return 3, rinse 1, season 1, serve
8, sprinkle 1, taste 1, test 1, Toss 8, try 3, ...
Figure 1.  Excepts of entries in the collocation database for
eat  and salad .
Table 1 . The top 20 most similar words of eat and salad  as
given by (Lin, 1998b).
W ORD S IMILAR W ORDS ( WITH SIMILARITY SCORE )
EAT cook 0.127, drink 0.108, consume 0.101, feed 0.094,
taste 0.093, like 0.092, serve 0.089, bake 0.087, sleep
0.086, pick 0.085, fry 0.084, freeze 0.081, enjoy
0.079, smoke 0.078, harvest 0.076, love 0.076, chop
0.074, sprinkle 0.072, Toss 0.072, chew 0.072
SALAD soup 0.172, sandwich 0.169, sauce 0.152, pasta
0.149, dish 0.135, vegetable 0.135, cheese 0.132,
dessert 0.13, entree 0.121, bread 0.116, meat 0.116,
chicken 0.115, pizza 0.114, rice 0.112, seafood 0.11,
dressing 0.109, cake 0.107, steak 0.105, noodle
0.105, bean 0.102
the collocation database for the words eat  and
salad . The database contains a total of 11
million unique dependency relationships.
2.2 Corpus-based thesaurus
Using the collocation database, Lin (1998b) used
an unsupervised method to construct a corpus-
based thesaurus consisting of 11839 nouns, 3639
verbs and 5658 adjectives/adverbs. Given a
word w, the thesaurus returns a set of similar
words of w  along with their similarity to w . For
example, the 20 most similar words of eat and
salad are shown in Table 1.
3 Training Data Extraction
We parsed a 125-million word newspaper
corpus with Minipar 3 , a descendent of Principar
(Lin, 1994). Minipar outputs dependency trees
(Lin, 1999) from the input sentences. For
example, the following sentence is decomposed
into a dependency tree:
Occasionally, the parser generates incorrect
dependency trees. For example, in the above
sentence, the prepositional phrase headed by
with  should attach to saw  (as opposed to dog ).
Two separate sets of training data were then
extracted from this corpus. Below, we briefly
describe how we obtained these data sets.
3.1 Ambiguous Data Set
For each input sentence, Minipar outputs a
single dependency tree. For a sentence
containing one or more prepositions, we use a
program to detect any alternative prepositional
attachment sites. For example, in the above
sentence, the program would detect that with
could attach to saw . Using an iterative
algorithm, we initially create a table of co-
occurrence frequencies for 3-tuples of the form
( V , P , N 2 ) and ( N 1 , P , N 2 ). For each k  possible
attachment site of a preposition P , we increment
the frequency of the corresponding 3-tuple by
1/ k . For example, Table 2 shows the initial co-
occurrence frequency table for the
corresponding 3-tuples of the above sentence.
                                                  
3 Available at www.cs. ualberta .ca/~lindek/minipar.htm.
In the following iterations of the algorithm, we
update the frequency table as follows. For each k
possible attachment site of a preposition P , we
refine its attachment score using the formulas
described in Section 4: VScore ( V k ,  P k ,  N 2 k ) and
NScore ( N 1 k ,  P k ,  N 2 k ). For any tuple ( W k , P k ,  N 2 k ),
where W k  is either V k  or N 2 k , we update its
frequency as:
( ) ( )( )∑
=
= k
i iii
kkk
kNkPkW NPWScore
NPWScorefr
1 2
2
2,, ,,
,,
where Score ( W k , P k ,  N 2 k ) = VScore ( W k , P k ,  N 2 k )
if W k  = V k ; otherwise Score ( W k , P k ,  N 2 k ) =
NScore ( W k , P k ,  N 2 k ).
Suppose that after the initial frequency table is
set NScore ( man, in, park ) = 1.23, VScore ( saw,
with, telescope ) = 3.65, and NScore ( dog, with,
telescope ) = 0.35. Then, the updated co-
occurrence frequencies for ( man , in , park ) and
( saw , with ,  telescope ) are:
fr ( man , in , park ) =    23.1 23.1 = 1.0
fr ( saw , with, telescope ) = 35.065.3 65.3 + = 0.913
Table 3  shows the updated frequency table
after the first iteration of the algorithm. The
resulting database contained 8,900,000 triples.
3.2 Unambiguous Data Set
As in ( Ratnaparkhi, 1998 ), we constructed a
training data set consisting of only unambiguous
Table 2 . Initial co-occurrence  frequency table entries for A
man in the park saw a dog with a telescope .
V OR N 1 P N 2 F REQUENCY
man in park 1.0
saw with telescope 0.5
dog with telescope 0.5
Table 3 . Co-occurrence  frequency table entries for A man
in the park saw a dog with a telescope  after one iteration.
V OR N 1 P N 2 F REQUENCY
man in park 1.0
saw with telescope 0.913
dog with telescope 0.087
A  man  in  the  park  saw  a  dog  with  a  telescope.
det det det det
pcomppcomp
mod
subj
obj
mod
attachments of the form ( V , P , N 2 ) and ( N 1 , P ,
N 2). We only extract a 3-tuple from a sentence
when our program finds no alternative
attachment site for its preposition. Each
extracted 3-tuple is assigned a frequency count
of 1. For example, in the previous sentence,
( man , in , park ) is extracted since it contains only
one attachment site; ( dog , with , telescope ) is not
extracted since with  has an alternative
attachment site. The resulting database
contained 4,400,000 triples.
4 Classification Model
Roth (1998) presented a unified framework for
natural  disambiguation tasks.
Essentially, several  learning algorithms
(e.g. naïve Bayes estimation, back-off
estimation, transformation-based learning) were
successfully cast as learning linear separators in
their feature space. Roth modelled prepositional
phrase attachment as linear combinations of
features. The features consisted of all 15
possible sub-sequences of the 4-tuple ( V , N 1 , P,
N 2 ) shown in Table 4. The asterix ( * ) in features
represent wildcards.
Roth used supervised learning to adjust the
weights of the features. In our experiments, we
only considered features that contained P  since
the preposition is the most important lexical item
(Collins and Brooks, 1995). Furthermore, we
omitted features that included both V  and N 1
since their co-occurrence is independent of the
attachment decision. The resulting subset of
features considered in our system is shown in
bold in Table 4 (equivalent to assigning a weight
of 0 or 1 to each feature).
Let | head , rel , mod | represent the frequency,
obtained from the training data, of the head
occurring in the given relationship rel  with the
modifier . We then assign a score to each feature
as follows:
1.  ( * , * , P , * ) = log (| * , P , * | / | * , * , * |)
2.  ( V , * , P , N 2 ) = log (| V , P , N 2 | / | * , * , * |)
3.  ( * , N 1 , P , N 2 ) = log (| N 1 , P , N 2 | / | * , * , * |)
4.  ( V , * , P , * ) = log (| V , P , * | / | V , * , * |)
5.  ( * , N 1 , P , * ) = log (| N 1 , P , * | / | N 1 , * , * |)
6.  ( * , * , P , N 2 ) = log (| * , P , N 2 | / | * , * , N 2 |)
1, 2, and 3 are the prior probabilities of P , V P
N 2, and N 1  P  N 2 respectively. 4, 5, and 6
represent conditional probabilities P( V , P  | V ),
P( N 1 , P  | N 1 ), and P( P N 2  | N 2 ) respectively.
We estimate the adverbial and adjectival
attachment scores, VScore ( V , P , N 2) and
NScore ( N 1 , P , N 2), as a linear combination of
these features:
VScore ( V , P , N 2 ) = ( * , * , P , * ) + ( V , * , P , N 2 ) +
( V , * , P , * ) + ( * , * , P , N 2 )
NScore ( N 1 , P , N 2 ) = ( * , * , P , * ) + ( * , N 1 , P , N 2 ) +
( * , N 1 , P , * ) + ( * , * , P , N 2 )
For example, the attachment scores for ( eat ,
salad , with , fork ) are VScore ( eat , with , fork ) =
-3.47 and NScore ( salad , with , fork ) = -4.77. The
model correctly assigns a higher score to the
adverbial attachment.
5 Contextually Similar Words
The contextually similar words of a word w  are
words similar to the intended meaning of w  in its
context. Below, we describe an algorithm for
constructing contextually similar words and we
present a method for approximating the
attachment scores using these words.
5.1 Algorithm
For our purposes, a context of w  is simply a
dependency relationship involving w . For
example, a dependency relationship for saw  in
the example sentence of Section 3 is
saw : obj : dog .  Figure 2 gives the data flow
diagram for our algorithm for constructing the
contextually similar words of w . We retrieve
from the collocation database the words that
occurred in the same dependency relationship as
w . We refer to this set of words as the cohort  of
w  for the dependency relationship. Consider the
words eat and salad in the context eat salad .
The cohort of eat  consists of verbs that appeared
Table 4 . The 15 features for prepositional phrase
attachment.
F EATURES
( V , * , * , * ) ( V , * , P , * ) ( * , N 1 , * , N 2 )
( V , N 1 , * , * ) ( V , * , * , N 2 ) ( * , N 1 , P , N 2 )
( V , N 1 , P , * ) ( V , * , P , N 2 ) ( * , * , P , * )
( V , N 1 , * , N 2 ) ( * , N 1 , * , * ) ( * , * , * , N 2 )
( V , N 1 , P , N 2 ) ( * , N 1 , P , * ) ( * , * , P , N 2 )
with object salad  in Figure 1 (e.g. add, consume,
cover, … ) and the cohort of salad  consists of
nouns that appeared as object of eat  in Figure 1
(e.g. almond, apple, bean,  …).
Intersecting the set of similar words and the
cohort then forms the set of contextually similar
words of w. For example, Table 5 shows the
contextually similar words of eat  and salad  in
the context eat salad  and the contextually
similar words of fork  in the contexts eat with
fork  and salad with fork . The words in the first
row are retrieved by intersecting the similar
words of eat  in  Table 1 with the cohort of eat
while the second row represents the intersection
of the similar words of salad  in Table 1 and the
cohort of salad . The third and fourth rows are
determined in a similar manner. In the
nonsensical context salad with fork  (in row 4),
no contextually similar words are found.
While previous word sense disambiguation
algorithms rely on a lexicon to provide sense
inventories of words, the contextually similar
words provide a way of distinguishing between
different senses of words without committing to
any particular sense inventory.
5.2 Attachment Approximation
Often, sparse data reduces our confidence in the
attachment scores of Section 4. Using
contextually similar words, we can approximate
these scores. Given the tuple ( V , N 1 , P , N 2 ),
adverbial attachments are approximated as
follows. We first construct a list CS V  containing
the contextually similar words of V  in context
V : obj : N 1 and a list CS N 2 V  containing the
contextually similar words of N 2 in context
V : P : N 2  (i.e. assuming adverbial attachment). For
each verb v  in CS V , we compute VScore ( v , P , N 2 )
and set S V  as the average of the largest k  of these
scores. Similarly, for each noun n  in CS N 2 V , we
compute VScore ( V , P , n ) and set S N 2 V  as the
average of the largest k  of these scores. Then,
the approximated adverbial attachment score,
Vscore' , is:
VScore' ( V , P , N 2 ) = max ( S V , S N 2 V )
We approximate the adjectival attachment
score in a similar way. First, we construct a list
CS N 1 containing the contextually similar words
of N 1 in context V : obj : N 1 and a list CS N 2 N 1
containing the contextually similar words of N 2
in context N 1 : P : N 2 (i.e. assuming adjectival
attachment). Now, we compute S N 1 as the
average of the largest k  of NScore ( n , P , N 2 ) for
each noun n  in CS N 1  and S N 2 N 1  as the average of
the largest k  of NScore ( N 1 , P , n ) for each noun n
in CS N 2 N 1. Then, the approximated adjectival
attachment score, NScore' , is:
NScore' ( N 1 , P , N 2 ) = max ( S N 1 , S N 2 N 1 )
For example, suppose we wish to approximate
the attachment score for the 4-tuple ( eat , salad ,
with , fork ). First, we retrieve the contextually
similar words of eat  and salad  in context eat
salad , and the contextually similar words of fork
in contexts eat with fork  and salad with fork  as
shown in Table 5 . Let k  = 2. Table 6  shows the
calculation of S V  and S N 2 V  while the calculation
of S N 1  and S N 2 N 1  is shown in Table 7 . Only the
Figure 2.  Data flow diagram for identifying the
contextually similar words of a word in a dependency
relationship.
word in dependency
relationship
Similar Words Cohorts
Corpus-Based
Thesaurus
Retrieve
Intersect
Get Similar
Words
Collocation
DB
Contextually
Similar Words
Table 5 . Contextually similar words of eat and salad .
W ORD C ONTEXT C ONTEXTUALLY S IMILAR W ORDS
EAT eat salad consume, taste, like, serve, pick,
harvest, love, sprinkle, Toss,  …
SALAD eat salad soup, sandwich, pasta, dish, cheese,
vegetable, bread, meat, cake, bean,  …
FORK eat with fork spoon, knife, finger
FORK salad with fork ---
top k = 2 scores are shown in these tables. We
have:
VScore'  ( eat , with , fork ) = max ( S V , S N 2 V )
= -2.92
NScore'  ( salad , with , fork ) = max ( S N 1 , S N 2 N 1 )
= -4.87
Hence, the approximation correctly prefers the
adverbial attachment to the adjectival
attachment.
6 Attachment Algorithm
Figure 3 describes the prepositional phrase
attachment algorithm. As in previous
approaches, examples with P  = of  are always
classified as adjectival attachments.
Suppose we wish to approximate the
attachment score for the 4-tuple ( eat , salad ,
with , fork ). From the previous section, Step 1
returns average V  = -2.92 and average N 1  = -4.87.
From Section 4, Step 2 gives a V  = -3.47 and
a N 1 = -4.77. In our training data, f V  = 2.97 and
f N 1  = 0, thus Step 3 gives f  = 0.914. In Step 4, we
compute:
S ( V ) = -3.42 and
S ( N 1 ) = -4.78
Since S ( V ) > S ( N 1), the algorithm correctly
classifies this example as an adverbial
attachment.
Given the 4-tuple ( eat , salad , with , croutons ),
the algorithm returns S ( V ) = -4.31 and S ( N 1 ) =
-3.88. Hence, the algorithm correctly attaches
the prepositional phrase to the noun salad .
7 Experimental Results
In this section, we describe our test data and the
baseline for our experiments. Finally, we present
our results.
7.1 Test Data
The test data consists of 3097 examples derived
from the manually annotated attachments in the
Penn Treebank Wall Street Journal data
(Ratnaparkhi et al., 1994) 4 . Each line in the test
data consists of a 4-tuple and a target
classification: V  N 1  P  N 2  target .
                                                  
4 Available at ftp.cis.upenn.edu/pub/adwait/PPattachData.
The data set contains several erroneous tuples
and attachments. For instance, 133 examples
contain the word the  as N 1  or N 2 . There are also
improbable attachments such as ( sing ,  birthday ,
to ,  you ) with the target attachment birthday .
Table 6 . Calculation of S V  and S N 2 V  for ( eat , salad , with ,
fork ).
4- TUPLE VS CORE
(mix, salad, with, fork) -2.60
(sprinkle, salad, with, fork) -3.24
S V -2.92
(eat, salad, with, spoon) -3.06
(eat, salad, with, finger) -3.50
S N 2 V -3.28
Table 7 . Calculation of S N 1  and S N 2 N 1  for (eat, salad, with,
fork).
4- TUPLE NS CORE
(eat, pasta, with, fork) -4.71
(eat, cake, with, fork) -5.02
S N 1 -4.87
--- n/a
--- n/a
S N 2 N 1 n/a
Input : A 4-tuple ( V , N 1 , P , N 2 )
Step 1 : Using the contextually similar words algorithm
and the formulas from Section 5.2 compute:
average V = VScore' ( V , P , N 2 )
average N 1 = NScore' ( N 1 , P , N 2 )
Step 2 : Compute the adverbial attachment score, a v ,
and the adjectival attachment score, a n 1 :
a V = VScore ( V , P , N 2 )
a N 1 = NScore ( N 1 , P , N 2 )
Step 3 : Retriev e from the training data set the
frequency of the 3-tuples ( V , P , N 2 ) and
( N 1 , P , N 2 ) � f V  and f N 1 , respectively.
Let f  = ( f V  + f N 1  + 0.2) / ( f V  + f N 1  +0.5)
Step 4 : Combine the scores of Steps 1-3 to obtain the
final attachment scores:
S ( V ) = fa v  + (1 − f ) average v
S ( N 1 ) = fa n 1  + (1 − f ) average n 1
Output : The attachment decision: N 1  if S ( N 1 ) > S ( V ) or
P  = of ; V  otherwise.
Figure 3.  The prepositional phrase attachment algorithm.
7.2 Baseline
Choosing the most common attachment site, N 1 ,
yields an accuracy of 58.96%. However, we
achieve 70.39% accuracy by classifying each
occurrence of P  = of  as N 1 , and V  otherwise.
Human accuracy, given the full context of a
sentence, is 93.2% and drops to 88.2% when
given only tuples of the form ( V , N 1 , P , N 2 )
(Ratnaparkhi et al., 1994). Assuming that human
accuracy is the upper bound for automatic
methods, we expect our accuracy to be bounded
above by 88.2% and below by 70.39%.
7.3 Results
We used the 3097-example testing corpus
described in Section 7.1. Table 8 presents the
precision and recall of our algorithm and Table 9
presents a performance comparison between our
system and previous supervised and
unsupervised approaches using the same test
data. We describe the different classifiers below:
cl base : the baseline described in Section 7.2
cl R1 : uses a maximum entropy model
(Ratnaparkhi et al., 1994)
cl BR 5 : uses transformation-based learning (Brill
and Resnik, 1994)
cl CB : uses a backed-off model (Collins and
Brooks, 1995)
cl SN : induces a decision tree with a sense-tagged
corpus, using a semantic dictionary
(Stetina and Nagao, 1997)
cl HR 6 : uses lexical preference (Hindle and Rooth,
1993)
cl R2 : uses a heuristic extraction of unambiguous
attachments (Ratnaparkhi, 1998)
cl PL : uses the algorithm described in this paper
Our classifier outperforms all previous
unsupervised techniques and approaches the
performance of supervised algorithm.
We reconstructed the two earlier unsupervised
classifiers cl HR  and cl R2 . Table 10  presents the
accuracy of our reconstructed classifiers. The
originally reported accuracy for cl R 2  is within the
95% confidence interval of our reconstruction.
Our reconstruction of cl HR  achieved slightly
higher accuracy than the original report.
                                                  
5 The accuracy is reported in (Collins and Brooks, 1995).
6 The accuracy was obtained on a smaller test set but, from
the same source as our test data.
Our classifier used a mixture of the two
training data sets described in Section 3. In
Table 11 , we compare the performance of our
system on the following training data sets:
UNAMB : the data set of unambiguous examples
described in Section 3.2
EM0 : the data set of Section 3.1 afte r
frequency table initialization
EM1 : EM0  + one iteration of algorithm 3.1
EM2 : EM0  + two iterations of algorithm 3.1
EM3 : EM0  + three iterations of algorithm 3.1
1/8-EM1 : one eighth of the data in EM1
MIX : The concatenation of UNAMB and EM1
Table 11 illustrates a slight but consistent
increase in performance when using contextually
similar words. However, since the confidence
intervals overlap, we cannot claim with certainty
Table 8 . Precision and recall for attachment sites V  and N 1 .
C LASS A CTUAL C ORRECT I NCORRECT P RECISION R ECALL
V 1203 994 209 82.63% 78.21%
N 1 1894 1617 277 84.31% 88.55%
Table 9 . Performance comparison with other approaches.
M ETHOD L EARNING A CCURACY
CL BASE --- 70.39%
CL R1 supervised 81.6%
CL BR supervised 81.9%
CL CB supervised 84.5%
CL SN supervised 88.1%
CL HR unsupervised 75.8%
CL R2 unsupervised 81.91%
CL PL unsupervised 84.31%
Table 10 . Accuracy of our reconstruction of (Hindle &
Rooth, 1993) and (Ratnaparkhi, 1998).
METHOD O RIGINAL
R EPORTED
A CCURACY
R ECONSTRUCTED
S YSTEM A CCURACY
(95% CONF )
CL HR 75.8% 78.40% ± 1.45%
CL R2 81.91% 82.40% ± 1.34%
that the contextually similar words improve
performance.
In Section 7.1, we mentioned some testing
examples contained N 1  = the  or N 2  = the . For
supervised algorithms, the  is represented in the
training set as any other noun. Consequently,
these algorithms collect training data for the  and
performance is not affected. However,
unsupervised methods break down on such
examples. In Table 12 , we illustrate the
performance increase of our system when
removing these erroneous examples.
Conclusion and Future Work
The algorithms presented in this paper advance
the state of the art for unsupervised approaches
to prepositional phrase attachment and draws
near the performance of supervised methods.
Currently, we are exploring different functions
for combining contextually similar word
approximations with the attachment scores. A
promising approach considers the mutual
information between the prepositional
relationship of candidate attachments and N 2 . As
the mutual information decreases, our
confidence in the attachment score decreases
and the contextually similar word approximation
is weighted higher. Also, improving the
construction algorithm for contextually similar
words would possibly improve the accuracy of
the system. One approach first clusters the
similar words. Then, dependency relationships
are used to select the most representative
clusters as the contextually similar words. The
assumption is that more representative similar
words produce better approximations.
Acknowledgements
The authors wish to thank the reviewers for their
helpful comments. This research was partly
supported by Natural Sciences and Engineering
Research Council of Canada grant OGP121338
and scholarship PGSB207797.
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Table 11 . Performance comparison of different data sets.
D ATABASE A CCURACY
WITHOUT
S IMWORDS
(95% CONF )
A CCURACY
WITH
S IMWORDS
(95% CONF )
U NAMBIGUOUS 83.15% ± 1.32% 83.60% ± 1.30%
EM0 82.24% ± 1.35% 82.69% ± 1.33%
EM1 83.76% ± 1.30% 83.92% ± 1.29%
EM2 83.66% ± 1.30% 83.70% ± 1.31%
EM3 83.20% ± 1.32% 83.20% ± 1.32%
1/8-EM1 82.98% ± 1.32% 83.15% ± 1.32%
MIX 84.11% ± 1.29% 84.31% ± 1.28%
Table 12 . Performance with removal of the  as N 1  or N 2 .
D ATA S ET A CCURACY
WITHOUT
S IMWORDS
(95% CONF )
A CCURACY
WITH
S IMWORDS
(95% CONF )
WITH THE 84.11% ± 1.29% 84.31% ± 1.32%
WITHOUT THE 84.44% ± 1.31% 84.65% ± 1.30%
