Parsing the WSJ using CCG and Log-Linear Models
Stephen Clark
School of Informatics
University of Edinburgh
2 Buccleuch Place, Edinburgh, UK
stephen.clark@ed.ac.uk
James R. Curran
School of Information Technologies
University of Sydney
NSW 2006, Australia
james@it.usyd.edu.au
Abstract
This paper describes and evaluates log-linear
parsing models for Combinatory Categorial
Grammar (CCG). A parallel implementation of
the L-BFGS optimisation algorithm is described,
which runs on a Beowulf cluster allowing the
complete Penn Treebank to be used for estima-
tion. We also develop a new efficient parsing
algorithm for CCG which maximises expected
recall of dependencies. We compare models
which use all CCG derivations, including non-
standard derivations, with normal-form models.
The performances of the two models are com-
parable and the results are competitive with ex-
isting wide-coverage CCG parsers.
1 Introduction
A number of statistical parsing models have recently
been developed for Combinatory Categorial Gram-
mar (CCG; Steedman, 2000) and used in parsers ap-
plied to the WSJ Penn Treebank (Clark et al., 2002;
Hockenmaier and Steedman, 2002; Hockenmaier,
2003b). In Clark and Curran (2003) we argued
for the use of log-linear parsing models for CCG.
However, estimating a log-linear model for a wide-
coverage CCG grammar is very computationally ex-
pensive. Following Miyao and Tsujii (2002), we
showed how the estimation can be performed effi-
ciently by applying the inside-outside algorithm to
a packed chart. We also showed how the complete
WSJ Penn Treebank can be used for training by de-
veloping a parallel version of Generalised Iterative
Scaling (GIS) to perform the estimation.
This paper significantly extends our earlier work
in a number of ways. First, we evaluate a number
of log-linear models, obtaining results which are
competitive with the state-of-the-art for CCG pars-
ing. We also compare log-linear models which use
all CCG derivations, including non-standard deriva-
tions, with normal-form models. Second, we find
that GIS is unsuitable for estimating a model of the
size being considered, and develop a parallel ver-
sion of the L-BFGS algorithm (Nocedal and Wright,
1999). And finally, we show that the parsing algo-
rithm described in Clark and Curran (2003) is ex-
tremely slow in some cases, and suggest an efficient
alternative based on Goodman (1996).
The development of parsing and estimation algo-
rithms for models which use all derivations extends
existing CCG parsing techniques, and allows us to
test whether there is useful information in the addi-
tional derivations. However, we find that the perfor-
mance of the normal-form model is at least as good
as the all-derivations model, in our experiments to-
date. The normal-form approach allows the use of
additional constraints on rule applications, leading
to a smaller model, reducing the computational re-
sources required for estimation, and resulting in an
extremely efficient parser.
This paper assumes a basic understanding of
CCG; see Steedman (2000) for an introduction, and
Clark et al. (2002) and Hockenmaier (2003a) for an
introduction to statistical parsing with CCG.
2 Parsing Models for CCG
CCG is unusual among grammar formalisms in that,
for each derived structure for a sentence, there can
be many derivations leading to that structure. The
presence of such ambiguity, sometimes referred to
as spurious ambiguity, enables CCG to produce el-
egant analyses of coordination and extraction phe-
nomena (Steedman, 2000). However, the introduc-
tion of extra derivations increases the complexity of
the modelling and parsing problem.
Clark et al. (2002) handle the additional deriva-
tions by modelling the derived structure, in their
case dependency structures. They use a conditional
model, based on Collins (1996), which, as the au-
thors acknowledge, has a number of theoretical de-
ficiencies; thus the results of Clark et al. provide a
useful baseline for the new models presented here.
Hockenmaier (2003a) uses a model which
favours only one of the derivations leading to a
derived structure, namely the normal-form deriva-
tion (Eisner, 1996). In this paper we compare the
normal-form approach with a dependency model.
For the dependency model, we define the probabil-
ity of a dependency structure as follows:
P( jS )=
X
d2 ( )
P(d; jS ) (1)
where  is a dependency structure, S is a sentence
and  ( ) is the set of derivations which lead to  .
This extends the approach of Clark et al. (2002)
who modelled the dependency structures directly,
not using any information from the derivations. In
contrast to the dependency model, the normal-form
model simply defines a distribution over normal-
form derivations.
The dependency structures considered in this pa-
per are described in detail in Clark et al. (2002)
and Clark and Curran (2003). Each argument slot
in a CCG lexical category represents a dependency
relation, and a dependency is defined as a 5-tuple
hh f; f; s;ha;li, where h f is the head word of the lex-
ical category, f is the lexical category, s is the argu-
ment slot, ha is the head word of the argument, and
l indicates whether the dependency is long-range.
For example, the long-range dependency encoding
company as the extracted object of bought (as in the
company that IBM bought) is represented as the fol-
lowing 5-tuple:
hbought;(S[dcl]nNP1)=NP2;2;company; i
where  is the category (NPnNP)=(S[dcl]=NP) as-
signed to the relative pronoun. For local dependen-
cies l is assigned a null value. A dependency struc-
ture is a multiset of these dependencies.
3 Log-Linear Parsing Models
Log-linear models (also known as Maximum En-
tropy models) are popular in NLP because of the
ease with which discriminating features can be in-
cluded in the model. Log-linear models have been
applied to the parsing problem across a range of
grammar formalisms, e.g. Riezler et al. (2002) and
Toutanova et al. (2002). One motivation for using
a log-linear model is that long-range dependencies
which CCG was designed to handle can easily be en-
coded as features.
A conditional log-linear model of a parse !2 ,
given a sentence S , is defined as follows:
P(!jS )= 1Z
S
e :f (!) (2)
where  :f(!) = Pi  i fi(!). The function fi is a
feature of the parse which can be any real-valued
function over the space of parses  . Each feature
fi has an associated weight  i which is a parameter
of the model to be estimated. ZS is a normalising
constant which ensures that P(!jS ) is a probability
distribution:
ZS =
X
!02 (S )
e :f (!0) (3)
where  (S ) is the set of possible parses for S .
For the dependency model a parse, !, is ahd; i
pair (as given in (1)). A feature is a count of the
number of times some configuration occurs in d or
the number of times some dependency occurs in  .
Section 6 gives examples of features.
3.1 The Dependency Model
We follow Riezler et al. (2002) in using a discrimi-
native estimation method by maximising the condi-
tional likelihood of the model given the data. For the
dependency model, the data consists of sentences
S 1;:::;S m, together with gold standard dependency
structures,  1;:::; m. The gold standard structures
are multisets of dependencies, as described earlier.
Section 6 explains how the gold standard structures
are obtained.
The objective function of a model is the condi-
tional log-likelihood, L( ), minus a Gaussian prior
term, G( ), used to reduce overfitting (Chen and
Rosenfeld, 1999). Hence, given the definition of the
probability of a dependency structure (1), the objec-
tive function is as follows:
L0( ) = L( ) G( ) (4)
= log
mY
j=1
P ( jjS j) 
nX
i=1
 2i
2 2i
=
mX
j=1
log
P
d2 ( j) e :f (d; j)
P
!2 (S j) e :f (!)
 
nX
i=1
 2i
2 2i
=
mX
j=1
log
X
d2 ( j)
e :f (d; j)
 
mX
j=1
log
X
!2 (S j)
e :f (!) 
nX
i=1
 2i
2 2i
where n is the number of features. Rather than have
a different smoothing parameter  i for each feature,
we use a single parameter  .
We use a technique from the numerical optimisa-
tion literature, the L-BFGS algorithm (Nocedal and
Wright, 1999), to optimise the objective function.
L-BFGS is an iterative algorithm which requires the
gradient of the objective function to be computed at
each iteration. The components of the gradient vec-
tor are as follows:
@L0( )
@ i =
mX
j=1
X
d2 ( j)
e :f (d; j) fi(d; j)
P
d2 ( j) e :f (d; j)
(5)
 
mX
j=1
X
!2 (S j)
e :f (!) fi(!)
P
!2 (S j) e :f (!)
  i 2
i
The first two terms in (5) are expectations of fea-
ture fi: the first expectation is over all derivations
leading to each gold standard dependency struc-
ture; the second is over all derivations for each sen-
tence in the training data. Setting the gradient to
zero yields the usual maximum entropy constraints
(Berger et al., 1996), except that in this case the
empirical values are themselves expectations (over
all derivations leading to each gold standard depen-
dency structure). The estimation process attempts
to make the expectations equal, by putting as much
mass as possible on the derivations leading to the
gold standard structures.1 The Gaussian prior term
penalises any model whose weights get too large in
absolute value.
Calculation of the feature expectations requires
summing over all derivations for a sentence, and
summing over all derivations leading to a gold stan-
dard dependency structure. In both cases there can
be exponentially many derivations, and so enumer-
ating all derivations is not possible (at least for
wide-coverage automatically extracted grammars).
Clark and Curran (2003) show how the sum over
the complete derivation space can be performed ef-
ficiently using a packed chart and a variant of the
inside-outside algorithm. Section 5 shows how the
same technique can also be applied to all derivations
leading to a gold standard dependency structure.
3.2 The Normal-Form Model
The objective function and gradient vector for the
normal-form model are as follows:
L0( ) = L( ) G( ) (6)
= log
mY
j=1
P (d jjS j) 
nX
i=1
 2i
2 2i
@L0( )
@ i =
mX
j=1
fi(d j) (7)
 
mX
j=1
X
d2 (S j)
e :f(d) fi(d)
P
d2 (S j) e :f(d)
  i 2
i
1See Riezler et al. (2002) for a similar description in the
context of LFG parsing.
where d j is the the gold standard derivation for sen-
tence S j and  (S j) is the set of possible derivations
for S j. Note that the empirical expectation in (7) is
simply a count of the number of times the feature
appears in the gold-standard derivations.
4 Packed Charts
The packed charts perform a number of roles: they
are a compact representation of a very large num-
ber of CCG derivations; they allow recovery of the
highest scoring parse or dependency structure with-
out enumerating all derivations; and they represent
an instance of what Miyao and Tsujii (2002) call a
feature forest, which is used to efficiently estimate a
log-linear model. The idea behind a packed chart is
simple: equivalent chart entries of the same type, in
the same cell, are grouped together, and back point-
ers to the daughters indicate how an individual entry
was created. Equivalent entries form the same struc-
tures in any subsequent parsing.
Since the packed charts are used for model es-
timation and recovery of the highest scoring parse
or dependency structure, the features in the model
partly determine which entries can be grouped to-
gether. In this paper we use features from the de-
pendency structure, and features defined on the lo-
cal rule instantiations.2 Hence, any two entries with
identical category type, identical head, and identical
unfilled dependencies are equivalent. Note that not
all features are local to a rule instantiation; for ex-
ample, features encoding long-range dependencies
may involve words which are a long way apart in
the sentence.
For the purposes of estimation and finding the
highest scoring parse or dependency structure, only
entries which are part of a derivation spanning the
whole sentence are relevant. These entries can be
easily found by traversing the chart top-down, start-
ing with the entries which span the sentence. The
entries within spanning derivations form a feature
forest (Miyao and Tsujii, 2002). A feature forest 
is a tuplehC;D;R; ; iwhere:
a0 C is a set of conjunctive nodes;
a0 D is a set of disjunctive nodes;
a0 R D is a set of root disjunctive nodes;
a0  : D!2C is a conjunctive daughter function;
a0  : C !2D is a disjunctive daughter function.
The individual entries in a cell are conjunctive
nodes, and the equivalence classes of entries are dis-
2By rule instantiation we mean the local tree arising from
the application of a CCG combinatory rule.
hC;D;R; ; iis a packed chart / feature forest
G is a set of gold standard dependencies
Let c be a conjunctive node
Let d be a disjunctive node
deps(c) is the set of dependencies on node c
cdeps(c)=
(  1 if, for some  2deps(c); < G
jdeps(c)j otherwise
dmax(c)=
8>>
><
>>>:
 1 if cdeps(c)= 1
 1 if dmax(d)= 1 for some d2 (c)P
d2 (c) dmax(d)+cdeps(c) otherwise
dmax(d)=maxfdmax(c)jc2 (d)g
mark(d):
mark d as a correct node
foreach c2 (d)
if dmax(c)=dmax(d)
mark c as a correct node
foreach d02 (c)
mark(d0)
foreach dr 2R such that dmax. (dr)=jGj
mark(dr)
Figure 1: Finding nodes in correct derivations
junctive nodes. The roots of the CCG derivations
represent the root disjunctive nodes.3
5 Efficient Estimation
The L-BFGS algorithm requires the following val-
ues at each iteration: the expected value, and the
empirical expected value, of each feature (to calcu-
late the gradient); and the value of the likelihood
function. For the normal-form model, the empiri-
cal expected values and the likelihood can easily be
obtained, since these only involve the single gold-
standard derivation for each sentence. The expected
values can be calculated using the method in Clark
and Curran (2003).
For the dependency model, the computations of
the empirical expected values (5) and the likelihood
function (4) are more complex, since these require
sums over just those derivations leading to the gold
standard dependency structure. We will refer to
such derivations as correct derivations.
Figure 1 gives an algorithm for finding nodes in
a packed chart which appear in correct derivations.
cdeps(c) is the number of correct dependencies on
conjunctive node c, and takes the value  1 if there
are any incorrect dependencies on c. dmax(c) is
3A more complete description of CCG feature forests is
given in Clark and Curran (2003).
the maximum number of correct dependencies pro-
duced by any sub-derivation headed by c, and takes
the value 1 if there are no sub-derivations produc-
ing only correct dependencies. dmax(d) is the same
value but for disjunctive node d. Recursive defini-
tions for calculating these values are given in Fig-
ure 1; the base case occurs when conjunctive nodes
have no disjunctive daughters.
The algorithm identifies all those root nodes
heading derivations which produce just the cor-
rect dependencies, and traverses the chart top-down
marking the nodes in those derivations. The in-
sight behind the algorithm is that, for two conjunc-
tive nodes in the same equivalence class, if one
node heads a sub-derivation producing more cor-
rect dependencies than the other node (and each
sub-derivation only produces correct dependencies),
then the node with less correct dependencies cannot
be part of a correct derivation.
The conjunctive and disjunctive nodes appearing
in correct derivations form a new correct feature for-
est. The correct forest, and the complete forest con-
taining all derivations spanning the sentence, can be
used to estimate the required likelihood value and
feature expectations. Let E  fi be the expected value
of fi over the forest for model ; then the values
in (5) can be obtained by calculating E j fi for the
complete forest j for each sentence S j in the train-
ing data (the second sum in (5)), and also E j fi for
each forest  j of correct derivations (the first sum
in (5)):
@L( )
@ i =
mX
j=1
(E j fi E j fi) (8)
The likelihood in (4) can be calculated as follows:
L( )=
mX
j=1
(log Z j  log Z j) (9)
where log Z is the normalisation constant for .
6 Estimation in Practice
The gold standard dependency structures are pro-
duced by running our CCG parser over the
normal-form derivations in CCGbank (Hocken-
maier, 2003a). Not all rule instantiations in CCG-
bank are instances of combinatory rules, and not all
can be produced by the parser, and so gold standard
structures were created for 85.5% of the sentences
in sections 2-21 (33,777 sentences).
The same parser is used to produce the packed
charts. The parser uses a maximum entropy su-
pertagger (Clark and Curran, 2004) to assign lexical
categories to the words in a sentence, and applies the
CKY chart parsing algorithm described in Steedman
(2000). For parsing the training data, we ensure that
the correct category is a member of the set assigned
to each word. The average number of categories as-
signed to each word is determined by a parameter
in the supertagger. For the first set of experiments,
we used a setting which assigns 1.7 categories on
average per word.
The feature set for the dependency model con-
sists of the following types of features: dependency
features (with and without distance measures), rule
instantiation features (with and without a lexical
head), lexical category features, and root category
features. Dependency features are the 5-tuples de-
fined in Section 1. There are also three additional
dependency feature types which have an extra dis-
tance field (and only include the head of the lex-
ical category, and not the head of the argument);
these count the number of words (0, 1, 2 or more),
punctuation marks (0, 1, 2 or more), and verbs (0,
1 or more) between head and dependent. Lexi-
cal category features are word–category pairs at the
leaf nodes, and root features are headword–category
pairs at the root nodes. Rule instantiation features
simply encode the combining categories together
with the result category. There is an additional rule
feature type which also encodes the lexical head of
the resulting category. Additional generalised fea-
tures for each feature type are formed by replacing
words with their POS tags.
The feature set for the normal-form model is
the same except that, following Hockenmaier and
Steedman (2002), the dependency features are de-
fined in terms of the local rule instantiations, by
adding the heads of the combining categories to the
rule instantiation features. Again there are 3 addi-
tional distance feature types, as above, which only
include the head of the resulting category. We had
hoped that by modelling the predicate-argument de-
pendencies produced by the parser, rather than local
rule dependencies, we would improve performance.
However, using the predicate-argument dependen-
cies in the normal-form model instead of, or in ad-
dition to, the local rule dependencies, has not led to
an improvement in parsing accuracy.
Only features which occurred more than once in
the training data were included, except that, for the
dependency model, the cutoff for the rule features
was 9 and the counting was performed across all
derivations, not just the gold-standard derivation.
The normal-form model has 482,007 features and
the dependency model has 984,522 features.
We used 45 machines of a 64-node Beowulf clus-
ter to estimate the dependency model, with an av-
erage memory usage of approximately 550 MB for
each machine. For the normal-form model we were
able to reduce the size of the charts considerably by
applying two types of restriction to the parser: first,
categories can only combine if they appear together
in a rule instantiation in sections 2–21 of CCGbank;
and second, we apply the normal-form restrictions
described in Eisner (1996). (See Clark and Curran
(2004) for a description of the Eisner constraints.)
The normal-form model requires only 5 machines
for estimation, with an average memory usage of
730 MB for each machine.
Initially we tried the parallel version of GIS de-
scribed in Clark and Curran (2003) to perform
the estimation, running over the Beowulf cluster.
However, we found that GIS converged extremely
slowly; this is in line with other recent results in the
literature applying GIS to globally optimised mod-
els such as conditional random fields, e.g. Sha and
Pereira (2003). As an alternative to GIS, we have
implemented a parallel version of our L-BFGS code
using the Message Passing Interface (MPI) standard.
L-BFGS over forests can be parallelised, using the
method described in Clark and Curran (2003) to cal-
culate the feature expectations. The L-BFGS algo-
rithm, run to convergence on the cluster, takes 479
iterations and 2 hours for the normal-form model,
and 1,550 iterations and roughly 17 hours for the
dependency model.
7 Parsing Algorithm
For the normal-form model, the Viterbi algorithm is
used to find the most probable derivation. For the
dependency model, the highest scoring dependency
structure is required. Clark and Curran (2003) out-
lines an algorithm for finding the most probable de-
pendency structure, which keeps track of the high-
est scoring set of dependencies for each node in
the chart. For a set of equivalent entries in the
chart (a disjunctive node), this involves summing
over all conjunctive node daughters which head sub-
derivations leading to the same set of high scoring
dependencies. In practice large numbers of such
conjunctive nodes lead to very long parse times.
As an alternative to finding the most probable
dependency structure, we have developed an algo-
rithm which maximises the expected labelled re-
call over dependencies. Our algorithm is based on
Goodman’s (1996) labelled recall algorithm for the
phrase-structure PARSEVAL measures.
Let L be the number of correct dependencies in
 with respect to a gold standard dependency struc-
ture G; then the dependency structure,  max, which
maximises the expected recall rate is:
 max = arg max E(L =jGj) (10)
= arg max 
X
 i
P( ijS )j \ ij
where S is the sentence for gold standard depen-
dency structure G and  i ranges over the depen-
dency structures for S . This expression can be ex-
panded further:
 max = arg max 
X
 i
P( ijS )
X
 2 
1 if  2 i
= arg max 
X
 2 
X
 0j 2 0
P( 0jS )
= arg max 
X
 2 
X
d2 ( 0)j 2 0
P(djS ) (11)
The final score for a dependency structure  is a
sum of the scores for each dependency  in  ; and
the score for a dependency  is the sum of the proba-
bilities of those derivations producing  . This latter
sum can be calculated efficiently using inside and
outside scores:
 max =arg max 
X
 2 
1
ZS
X
c2C
 c c if  2deps(c)
(12)
where  c is the inside score and  c is the outside
score for node c (see Clark and Curran (2003)); C
is the set of conjunctive nodes in the packed chart
for sentence S and deps(c) is the set of dependen-
cies on conjunctive node c. The intuition behind
the expected recall score is that a dependency struc-
ture scores highly if it has dependencies produced
by high scoring derivations.4
The algorithm which finds  max is a simple vari-
ant on the Viterbi algorithm, efficiently finding a
derivation which produces the highest scoring set of
dependencies.
8 Experiments
Gold standard dependency structures were derived
from section 00 (for development) and section 23
(for testing) by running the parser over the deriva-
tions in CCGbank, some of which the parser could
not process. In order to increase the number of test
sentences, and to allow a fair comparison with other
CCG parsers, extra rules were encoded in the parser
(but we emphasise these were only used to obtain
4Coordinate constructions can create multiple dependencies
for a single argument slot; in this case the score for the multiple
dependencies is the average of the individual scores.
LP LR UP UR cat
Dep model 86.7 85.6 92.6 91.5 93.5
N-form model 86.4 86.2 92.4 92.2 93.6
Table 1: Results on development set; labelled and unla-
belled precision and recall, and lexical category accuracy
Features LP LR UP UR cat
RULES 82.6 82.0 89.7 89.1 92.4
+HEADS 83.6 83.3 90.2 90.0 92.8
+DEPS 85.5 85.3 91.6 91.3 93.5
+DISTANCE 86.4 86.2 92.4 92.2 93.6
FINAL 87.0 86.8 92.7 92.5 93.9
Table 2: Results on development set for the normal-
form models
the section 23 test data; they were not used to parse
unseen data as part of the testing). This resulted in
2,365 dependency structures for section 23 (98.5%
of the full section), and 1,825 (95.5%) dependency
structures for section 00.
The first stage in parsing the test data is to apply
the supertagger. We use the novel strategy devel-
oped in Clark and Curran (2004): first assign a small
number of categories (approximately 1.4) on aver-
age to each word, and increase the number of cate-
gories if the parser fails to find an analysis. We were
able to parse 98.9% of section 23 using this strategy.
Clark and Curran (2004) shows that this supertag-
ging method results in a highly efficient parser.
For the normal-form model we returned the de-
pendency structure for the most probable derivation,
applying the two types of normal-form constraints
described in Section 6. For the dependency model
we returned the dependency structure with the high-
est expected labelled recall score.
Following Clark et al. (2002), evaluation is by
precision and recall over dependencies. For a la-
belled dependency to be correct, the first 4 elements
of the dependency tuple must match exactly. For
an unlabelled dependency to be correct, the heads
of the functor and argument must appear together
in some relation in the gold standard (in any order).
The results on section 00, using the feature sets de-
scribed earlier, are given in Table 1, with similar
results overall for the normal-form model and the
dependency model. Since experimentation is easier
with the normal-form model than the dependency
model, we present additional results for the normal-
form model.
Table 2 gives the results for the normal-form
model for various feature sets. The results show
that each additional feature type increases perfor-
LP LR UP UR cat
Clark et al. 2002 81.9 81.8 90.1 89.9 90.3
Hockenmaier 2003 84.3 84.6 91.8 92.2 92.2
Log-linear 86.6 86.3 92.5 92.1 93.6
Hockenmaier(POS) 83.1 83.5 91.1 91.5 91.5
Log-linear (POS) 84.8 84.5 91.4 91.0 92.5
Table 3: Results on the test set
mance. Hockenmaier also found the dependencies
to be very beneficial — in contrast to recent results
from the lexicalised PCFG parsing literature (Gildea,
2001) — but did not gain from the use of distance
measures. One of the advantages of a log-linear
model is that it is easy to include additional infor-
mation, such as distance, as features.
The FINAL result in Table 2 is obtained by us-
ing a larger derivation space for training, created
using more categories per word from the supertag-
ger, 2.9, and hence using charts containing more
derivations. (15 machines were used to estimate this
model.) More investigation is needed to find the op-
timal chart size for estimation, but the results show
a gain in accuracy.
Table 3 gives the results of the best performing
normal-form model on the test set. The results
of Clark et al. (2002) and Hockenmaier (2003a)
are shown for comparison. The dependency set
used by Hockenmaier contains some minor differ-
ences to the set used here, but “evaluating” our test
set against Hockenmaier’s gives an F-score of over
97%, showing the test sets to be very similar. The
results show that our parser is performing signifi-
cantly better than that of Clark et al., demonstrating
the benefit of derivation features and the use of a
sound statistical model.
The results given so far have all used gold stan-
dard POS tags from CCGbank. Table 3 also gives the
results if automatically assigned POS tags are used
in the training and testing phases, using the C&C
POS tagger (Curran and Clark, 2003). The perfor-
mance reduction is expected given that the supertag-
ger relies heavily on POS tags as features.
More investigation is needed to properly com-
pare our parser and Hockenmaier’s, since there are
a number of differences in addition to the models
used: Hockenmaier effectively reads a lexicalised
PCFG off CCGbank, and is able to use all of the
available training data; Hockenmaier does not use
a supertagger, but does use a beam search.
Parsing the 2,401 sentences in section 23 takes
1.6 minutes using the normal-form model, and 10.5
minutes using the dependency model. The differ-
ence is due largely to the normal-form constraints
used by the normal-form parser. Clark and Curran
(2004) shows that the normal-form constraints sig-
nificantly increase parsing speed and, in combina-
tion with adaptive supertagging, result in a highly
efficient wide-coverage parser.
As a final oracle experiment we parsed the sen-
tences in section 00 using the correct lexical cate-
gories from CCGbank. Since the parser uses only a
subset of the lexical categories in CCGbank, 7% of
the sentences could not be parsed; however, the la-
belled F-score for the parsed sentences was almost
98%. This very high score demonstrates the large
amount of information in lexical categories.
9 Conclusion
A major contribution of this paper has been the de-
velopment of a parsing model for CCG which uses
all derivations, including non-standard derivations.
Non-standard derivations are an integral part of the
CCG formalism, and it is an interesting question
whether efficient estimation and parsing algorithms
can be defined for models which use all derivations.
We have answered this question, and in doing so
developed a new parsing algorithm for CCG which
maximises expected recall of dependencies.
We would like to extend the dependency model,
by including the local-rule dependencies which are
used by the normal-form model, for example. How-
ever, one of the disadvantages of the dependency
model is that the estimation process is already using
a large proportion of our existing resources, and ex-
tending the feature set will further increase the exe-
cution time and memory requirement of the estima-
tion algorithm.
We have also shown that a normal-form model
performs as well as the dependency model. There
are a number of advantages to the normal-form
model: it requires less space and time resources
for estimation and it produces a faster parser. Our
normal-form parser significantly outperforms the
parser of Clark et al. (2002) and produces results
at least as good as the current state-of-the-art for
CCG parsing. The use of adaptive supertagging and
the normal-form constraints result in a very efficient
wide-coverage parser. Our system demonstrates
that accurate and efficient wide-coverage CCG pars-
ing is feasible.
Future work will investigate extending the feature
sets used by the log-linear models with the aim of
further increasing parsing accuracy. Finally, the ora-
cle results suggest that further experimentation with
the supertagger will significantly improve parsing
accuracy, efficiency and robustness.
Acknowledgements
We would like to thank Julia Hockenmaier for
the use of CCGbank and helpful comments, and
Mark Steedman for guidance and advice. Jason
Baldridge, Frank Keller, Yuval Krymolowski and
Miles Osborne provided useful feedback. This work
was supported by EPSRC grant GR/M96889, and a
Commonwealth scholarship and a Sydney Univer-
sity Travelling scholarship to the second author.
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