3 Our Model
Similar to extractive and abstractive models, the
proposed event-driven model consists of two
steps, namely candidate extraction and headline
generation.
3.1 Candidate Extraction
We exploit events as the basic units for candidate
extraction. Here an event is a tuple (S, P, O),
where S is the subject, P is the predicate and O is
the object. For example, for the sentence “Ukraine
Delays Announcement of New Government”, the
event is (Ukraine, Delays, Announcement). This
type of event structures has been used in open
information extraction (Fader et al., 2011), and has
a range of NLP applications (Ding et al., 2014; Ng
et al., 2014).
A sentence is a well-formed structure with
complete syntactic information, but can contain
redundant information for text summarization,
which makes sentences very sparse. Phrases can
be used to avoid the sparsity problem, but with
little syntactic information between phrases, fluent
headline generation is difficult. Events can be
regarded as a trade-off between sentences and
phrases. They are meaningful structures without
redundant components, less sparse than sentences
and containing more syntactic information than
phrases.
In our system, candidate event extraction is
performed on a bipartite graph, where the two
types of nodes are lexical chains (Section 3.1.2)
and events (Section 3.1.1), respectively. Mutual
Reinforcement Principle (Zha, 2002) is applied
to jointly learn chain and event salience on the
bipartite graph for a given input. We obtain the
top-k candidate events by their salience measures.
3.1.1 Extracting Events
We apply an open-domain event extraction
approach. Different from traditional event
extraction, for which types and arguments are pre-
defined, open event extraction does not have a
closed set of entities and relations (Fader et al.,
2011). We follow Hu’s work (Hu et al., 2013) to
extract events.
Given a text, we first use the Stanford
dependency parser
1
to obtain the Stanford typed
dependency structures of the sentences (Marneffe
and Manning, 2008). Then we focus on
1
http://nlp.stanford.edu/software/lex-parser.shtml
DT NNPS MD VB DT NNP NNP POS NNS
the Keenans could demand the Aryan Nations ’ assets
nsubj
aux
dobj
det nn
poss
Figure 2: Dependency tree for the sentence
“the Keenans could demand the Aryan Nations’
assets”.
two relations, nsubj and dobj, for extracting
event arguments. Event arguments that have
the same predicate are merged into one event,
represented by tuple (Subject, Predicate, Object).
For example, given the sentence, “the Keenans
could demand the Aryan Nations’ assets”, Figure
2 present its partial parsing tree. Based
on the parsing results, two event arguments
are obtained: nsubj(demand, Keenans) and
dobj(demand, assets). The two event arguments
are merged into one event: (Keenans, demand,
assets).
3.1.2 Extracting Lexical Chains
Lexical chains are used to link semantically-
related words and phrases (Morris and Hirst, 1991;
Barzilay and Elhadad, 1997). A lexical chain is
analogous to a semantic synset. Compared with
words, lexical chains are less sparse for event
ranking.
Given a text, we follow Boudin and Morin
(2013) to construct lexical chains based on the
following principles:
1. All words that are identical after stemming
are treated as one word;
2. All NPs with the same head word fall into one
lexical chain;
2
3. A pronoun is added to the corresponding
lexical chain if it refers to a word in the chain
(The coreference resolution is performed
using the Stanford Coreference Resolution
system);
3
4. Lexical chains are merged if their main words
are in the same synset of WordNet.
4
2
NPs are extracted according to the dependency relations
nn and amod. As shown in Figure 2, we can extract the noun
phrase Aryan Nations according to the dependency relation
nn(Nations, Aryan).
3
http://nlp.stanford.edu/software/dcoref.shtml
4
http://wordnet.princeton.edu/