Invited talk at Copenhagen Dependency Treebank Workshop, Copenhagen, Denmark

1. Annotation of anaphora
and coreference for
automatic processing
Constantin Orasan
Research Group in Computational Linguistics
University of Wolverhampton, UK
http://www.wlv.ac.uk/~in6093/

2. Why use corpora in
anaphora/coreference resolution
 In this talk corpora discussed for:
 Training machine learning systems
 Testing anaphora/coreference resolution
algorithms
 Annotation:
 Linguistically motivated: tries to capture certain
phenomena (usually focuses on anaphora)
 Application motivated: limited relations are
encoded (usually focuses on coreference)

3. Structure
1. Background information
2. The MUC annotation for coreference
3. The NP4E corpus
4. Event coreference and NP coreference
5. Conclusions

4. Anaphora and anaphora
resolution
 cohesion which points back to some previous item
(Halliday and Hasan, 1976)
 the pointing back word is called an anaphor, the
entity to which it refers or for which it stands is its
antecedent (Mitkov, 2002)
 The process of determining the antecedent of an
anaphor is called anaphora resolution (Mitkov,
2002)
 Anaphora resolution can be seen as a process of
filling empty or almost empty expressions with
information from other expressions

5. Coreference and coreference
resolution
 When the anaphor refers to an antecedent
and when both have the same referent in real
world they are termed coreferential (Mitkov,
2002)
 The process of establishing which referential
NPs point to the same discourse entity is
called coreference resolution

6. Examples of anaphoric
expressions from Mitkov (2002)
Sophia Loren says she will always be grateful to
Bono. The actress revealed that the U2 singer helped
her calm down when she became scared by a
thunderstorm while travelling on a plane.
Coreferential chains:
 {Sophia Loren, she, the actress, her, she},
 {Bono, the U2 singer},
 {a thunderstorm},
 {a plane}

7. Examples of anaphoric
expressions from Mitkov (2002)
 Indirect anaphora: Although the store had only just
opened, the food hall was busy and there were long
queues at the tills.
 Identity-of-sense anaphora: The man who gave
his paycheck to his wife was wiser that the man who
gave it to his mistress
 Verb and adverb anaphora: Stephanie sang, as
did Mike
 Bound anaphora: Every man has his own agenda
 Cataphora: The elevator opened for him on the 14th
floor, and Alec stepped out quickly.

8. Anaphora vs. coreference
 There are many anaphoric expressions which are
not coreferential
 Most of the coreferential expressions are anaphoric
(Sophia Loren, the actress)
 Coreferential expressions that may be or may not be
anaphoric
 (Sophia Loren, the actress Sophia Loren) – not anaphoric?
 (the actress Sophia Loren, Sophia Loren) – anaphoric
 Coreferential expressions which are not anaphoric
(Sophia Loren, Sophia Loren)
 Cross-document coreference is not anaphora

9. Substitution test
 To determine whether two entities are
coreferential substitution test is used
 Sophia Loren says she will always be grateful to
Bono  Sophia Loren says Sophia Loren will
always be grateful to Bono.
 John has his own agenda  John has John’s own
agenda
 Every man has his own agenda.  Every man has
every man’s own agenda. ??

10. Anaphora & coreference in
computational linguistics
 are important preprocessing steps for a wide
range of applications such as machine
translation, information extraction, automatic
summarisation, etc.
 From linguistic perspective the expressions
processed are rather limited

11. Developing annotated corpora for
computational linguistics
A simple, reliable annotation task
Producing an CL-oriented resource
Capturing the most widespread and best-understood anaphoric
relation
identity-of-reference direct nominal anaphora
Including identity, Referring expressions (pronouns,
Elements synonymy, definite NPs, or proper names)
corresponding to the generalisation and have non-pronominal NP
same discourse entity specialisation antecedents in the preceding text /
dialogue

12. Terminology
 Entity = an object or set of objects in the world
 Entities can have types (ACE requires to annotate
only certain types e.g. person, location,
organisation, etc.)
 Mention = a textual reference to an entity (usually an
NP)
 Direct anaphora = identity of head, generalisation,
specialisation or synonymy
 Indirect anaphora = part-of, set-membership

13. Annotation of anaphora/
coreference
 In general the process can be split into two
stages:
 Identification and annotation of elements involved
in a relation (annotation of mentions)
 Identification and annotation of relations between
mentions
 The two stages can be done together or
separately

14. Annotation of mentions
 Annotate everything?
 Singletons should be annotated because they
influence evaluation measures (except MUC
score)
 If everything is annotated it is easier if this
annotation is done in the first instance
 Syntactic annotation can be useful

15. Annotation of relations
 Each annotation scheme defines a set of
relations that should be covered
 The relations normally happen between
mentions/markables

16. MUC annotation (Hirchmann
1997)
 Defined in the coreference resolution task at MUC
 The criteria used to define the task were:
1. Support for the MUC information extraction tasks;
2. Ability to achieve good (ca. 95%) interannotator
agreement;
3. Ability to mark text up quickly (and therefore, cheaply);
4. Desire to create a corpus for research on coreference and
discourse phenomena, independent of the MUC extraction
task.
 These criteria are not necessarily consistent with
each other

17. MUC annotation scheme
 Marks only relations between noun phrases
 Does not mark relations between verbs,
clauses, etc.
 Marks only IDENTITY which defines
equivalence classes and is not directional
 Values which are clearly distinct should not
be allowed to be in the same class e.g. the
stock price fell from $4.02 to $3.85

18. MUC annotation scheme (II)
 SGML used
<COREF ID="100">Lawson Mardon Group Ltd.</COREF> said
<COREF ID="101" TYPE="IDENT" REF="100">it </COREF> ...
 Attributes:
 ID a unique identifier for a mention
 REF indicates links between mentions
 TYPE the type of link (only IDENT supported)
 MIN the minimum span to be identified in order to be
considered correct in automatic evaluation
 STATUS=“OPT” to indicate optional elements to be
resolved

19. MUC annotation scheme –
markables (III)
 NPs (including dates, percentages and
currency expressions), personal and
demonstrative pronouns
 Interrogative “wh-” NPs are not marked
(Which engine would you like to use?)
 The extent of the markable is quite loosely
defined (must include the head, but should
really include the maximal NP and MIN
attribute have the head as the value)

20. MUC annotation scheme –
relations
 Basic coreference
 Bound anaphors
 Apposition
<COREF ID="1" MIN="Julius Caesar">Julius Caesar, <COREF
ID="2" REF="1" MIN="emperor" TYPE="IDENT"> the/a well-known
emperor,</COREF></COREF>
 Predicate nominals
<COREF ID="1" MIN="Julius Caesar">Julius Caesar</COREF> is
<COREF ID="2" REF="1" MIN="emperor" TYPE="IDENT">the/a
well-known emperor</COREF> who …
 For appositions and predicate nominals there needs
to be certainty (is not may be)

21. MUC annotation - criticism
 Van Deemter and Kibble (1999) criticised the
MUC scheme because it goes beyond
annotation of coreference as it is commonly
understood because:
 It marks quantifying NPs (e.g. every man, most
people)
 Marks indefinite NPs
Henry Higgins, who was formerly sales director of Sudsy
Soaps, became president of Dreamy Detergents.
 and one can argue not in a consistent manner
the stock price fell from $4.02 to $3.85

22. MUC annotation & corpus
 Despite criticism the MUC annotation provided a
starting point for standardising
anaphora/coreference annotation schemes
 Designed to mark only a small set of expressions
and relations which can be tackled by computers
 Was proposed in the context of a competition 
comparison of results and backing of an
organisation
 The corpus is available

23. Corpus of technical manuals
(Mitkov et. al. 2000)
 A corpus of technical manuals annotated with
a MUC-7 like annotation scheme
 Annotates only identity of reference between
direct nominal referential expressions
 Less interesting from linguistic perspective,
but used to develop automatic methods

24. Corpus of technical manuals
(Mitkov et. al. 2000)
 Full coreferential chains are annotated
 All the mentions are annotated regardless
whether they are singletons or not
 The relation of coreference is considered fully
transitive
 The MUC annotation scheme was used but
the guidelines were not adapted completely
 CLinkA (Orasan 2000) was used for
annotation

25. Annotation guidelines
 The starting point the MUC-7 annotation
guidelines, but
 More strict with what means identity of meaning
(e.g. we do not consider indefinite appositional
phrases coreferential with the phrases that
contain them)
 An indefinite NP cannot refer to anything
 Not consider gerunds as mentions

26.  Add missing phenomena:
 V [NP1] as [NP2] – not coreferential
[use [a diagonal linear gradient] as [the map]] – is not
coreferential
[elect [John Prescott] as [Prime Minister]], – is not coreferential
 …if [[ an NTSC Ø ]i or [ PAL monitor ]j]k is being used…[ The
NTSC monitor ]l… - not coreferential
…[[the pixels’ luminance]i or [Ø Ø saturation]j ]k is important…
[The pixels’ saturation]j - coreferential

27. Annotation guidelines – short version
Do: Do not:
(i) annotate identity-of-reference direct nominal (i) annotate indefinite predicate nominals that are linked to
anaphora other elements by perception verbs as coreferential with
those elements
(ii) annotate definite descriptions which stand in any of (ii) annotate identity-of-sense anaphora
the identity, synonymy, generalisation, specialisation, or
copula relationships with an antecedent
(iii) annotate definite NPs in a copula relation as (iii) annotate indirect anaphora between markables
coreferential
(iv) annotate definite appositional and bracketed phrases (iv) annotate cross-document coreference
as coreferential with the NP of which they are a part
(v) annotate NPs at all levels from base to complex and (v) annotate indefinite NPs in copula relations with other
co-ordinated NPs as coreferential
(vi) familiarise yourself with the use of unfamiliar, (vi) annotate non-permanent or “potential” coreference
highly specialised terminology by search through the between markables
text
(vii) annotate bound anaphors
(viii) consider gerunds of any kind markable
(ix) annotate anaphora over disjoined antecedents
(x) consider first or second person pronouns markable

28. Speed of annotation (Mitkov et.
al. 2000)
 Speed of annotation in one hour:
 At the beginning while the guidelines were being created:
assign 288 mentions to 220 entities covering on average
2051 words in text
 After the annotators became used to the task and the
guidelines finalised: assign 315 mentions to 250 entities
covering on average 1411 words in text
 Fast track annotation for pronoun resolution in one
hour: 113 pronouns, 944 candidates and 148
antecedents, covering 10862 words

29. Speed of annotation (II)
 Most of the time during the annotation is
spent identifying the mentions
 … existing annotation levels can prove very
beneficial

30. Reasons for disagreements
 The process is tedious and requires high
levels of concentration
 Two main reasons for disagreement:
 Unsteady references – mentions which may
belong to different entities through the document
(e.g. image, the window) – the automatic
annotation option of the annotation tool may also
mislead
 Specialised terminology

31. Improving annotation strategies
 Unsteady reference: Pre-annotation stage to clarify
topic segments
 Domain knowledge: Pre-annotation stage to
disambiguate unknown technical terminology
 ‘Master strategy’ combining individual
approaches:
 Printing text prior to annotation - increases familiarity
 Two step process
 Taking note of troublesome cases to discuss later with others
 Annotating intensively vs sporadically

32. NP4E (Hasler et. al. 2006)
 The goal was to develop annotation guidelines for
NP and event coreference in newspaper articles
about terrorism/security threats
 A small corpus annotated with NP and event
coreference was produced
 An attempt to produce a more refined annotated
resource than our previous corpus
 5 clusters of related documents in the domain were
built, about 50,000 words
 http://clg.wlv.ac.uk/projects/NP4E/

33. NP coreference annotation
 Used the guidelines developed by (Mitkov et. al.
2000) as the starting point,
 but adapted them for our goals and texts
 All the mentions need to be annotated, both definite and
indefinte NPs
 Introduced coref and ucoref tags to be able to deal with
uncertainties
The government] will argue that… [[McVeigh] and [Nichols]] were [the
masterminds of [the bombing plot]]
 Types of relations between an NP and its antecedent:
identity, synonymy, generalisation, specialisation and
other, but we do not annotate indirect anaphora

34. NP coreference annotation (II)
 Types of (coreference) relations we identify NP, copular, apposition,
bracketed text, speech pronoun and other
 Link to the first element of the chain in most of the cases for type NP
 For copular, apposition, bracketed text and speech pronouns (pronouns
which occur in direct speech), the anaphor should be linked back to the
nearest mention of the antecedent in the text
 Do not annotate coreferential different readings of an NP as
coreferential
[A jobless Taiwanese journalist who commandeered [a Taiwan airliner] to [China]]…
[China] ordered [[its] airports] to beef up [security]…

35. The user can
override WordNet is consulted
WordNet’s decision about the relation
between the two NPs
Annotation of NPs using PALinkA
the plane is marked as
coreferential with The
aircraft

36. Issues arising during the NP
annotation
 The antecedent of pronoun we in direct speech can
be linked to: the individual speaker, a group
represented by the speaker or nothing
 General concepts such as violence, terror, terrorism,
police, etc are sometimes used in a general sense
so it is difficult to know whether to annotate and how
 Sometimes difficult to decide the best indefinite NP
as an antecedent
…the man detained for hijacking [a Taiwanese airliner]… Liu
forced [a Far East Air Transport domestic plane]… Beijing
returned [the Boeing 757]…

37. Issues arising during the NP
annotation (II)
 Mark relative pronouns/clauses and link them
to the nearest mention
Chinese officials were tightlipped whether [Liu Shan-chung,
45, [who] is in custody in China's southeastern city of
Xiamen], would be prosecuted or repatriated to Taiwan.
 The type of relation is sometimes difficult to
establish without the help of WordNet (have
ident, non-ident)

38. Annotation of event coreference
 Event = a thing that happens or takes place, a single
specific occurrence, either instantaneous or
ongoing.
 Used the ACE annotation guidelines as starting
point
 Events marked: ATTACK, DEFEND, INJURE, DIE,
CONTACT
 Identify the trigger = the best word to represent the
event
 Triggers: verbs, nouns, adjectives and pronouns
{The blast} {killed} 168 people…and {injured}
hundreds more… (ATTACK: noun, DIE: verb,
INJURE: verb)

39. Event triggers
 ATTACK: attack events are physical actions which aim to cause harm
or damage tothings or people: attack, bomb, shoot, blast, war, fighting,
clashes, throw, hit, hold, spent.
 DEFEND: defend events are events where people or organisations
defend something, usually against someone or something else:
sheltering, reinforcing, running, prepared.
 INJURE: injure events involve people experiencing physical harm:
injure, hurt, maim, paralyse, wounded, ailing.
 DIE: die events happen when a person’s life ends: kill, dead, suicide,
fatal, assassinate, died, death.
 CONTACT: contact events occur when two or more parties
communicate in order to try and resolve something, reach an
agreement or better relations between different sides etc. This category
includes demands, threats and promises made by parties during
negotiations: meeting, talks, summit, met, negotiations, conference,
called, talked, phoned, discussed, promised, threatened, agree, reject,
demand.

40. Annotation of event coreference
 Two stage process: identify the triggers and then
link them
 Link arguments of an event to NP annotated in the
previous stage
 The arguments are event dependent (e.g.
ATTACKER, MEANS, VICTIM, CAUSE, AGENT,
TOPIC and MEDIUM)
 The arguments should be linked to NPs from the
same sentence or near by sentences if they are
necessary to disambiguate the event
 Also TENSE, MODALITY and POLARITY needs to
be indicated

41. Annotation of an attack event using PALinkA
the operation
TYPE: attack
TIME: Dec. 17
REF: stormed
TARGET: the Japanese
ambassador's residence in
Lima (FACILITY)
ATTACKER: MRTA rebels
(PERSON)
the operation PLACE: Lima (LOCATION)

42. Issues with event annotation
 Very difficult annotation task
 At times it is difficult to decide the tense of an event
in direct speech
 Whether to include demands, promises or threats in
the CONTACT (or use them only as a signal of
modality)
 Whether to make a distinction between
speaker/hearer in CONTACT events (especially in
the case of demands, promises or threats)

43. What coreferential events indicate?
(Hasler and Orasan 2009)
 Starting point – do coreferential events have
coreferential arguments?
 We had a corpus of about 12,000 words
annotated with event coreference
 344 unique event mentions
 106 coreferential chains with 2 to 10 triggers
 238 events referred by only one trigger

44. Zaire planes bombs rebels as U.N. seeks war’s end.
a293 TRIGGER: bombs
ATTACKER: –
MEANS: Zaire planes: ID=0: CHAIN=0: VEHICLE
PLACE: –
TARGET: rebels: ID=1: CHAIN=1: PERSON
TIME: –
Zaire said on Monday its warplanes were bombing three key rebel-held towns in its eastern
border provinces and that the raids would increase in intensity.
a333 TRIGGER: bombing
ATTACKER: Zaire: ID=44: CHAIN=5: ORGANISATION
MEANS: its warplanes: ID=46: CHAIN=46: VEHICLE
PLACE: three key rebel-held towns in its eastern border provinces: ID=48:
CHAIN=14: LOCATION
TARGET: three key rebel-held towns in its eastern border provinces: ID=48:
CHAIN=14: LOCATION
TIME: Monday: ID=45: CHAIN=7
“Since this morning the FAZ (Zaire army) has been bombing Bukavu, Shabunda and
Walikale”, said a defence ministry statement in the capital Kinshasa.
a334 TRIGGER: bombing
ATTACKER: the FAZ (Zaire army): ID=53: CHAIN=53: ORGANISATION
MEANS: –
PLACE: Bukavu, Shabunda and Walikale: ID=55: CHAIN=14: LOCATION
TARGET: Bukavu, Shabunda and Walikale: ID=55: CHAIN=14: LOCATION
TIME: this morning: ID=52: CHAIN=52

45. Referential relations between
arguments
 104 chains considered:
 22 (21.15%) contained only coferential NPs
 23 (22.12%) contained only non-coferential NPs
 9 chains ignored
 50 (48.07%) contain a mixture of coreferential and
non-coreferential NPs
 If indirect anaphora is not annotated, 70% of
chains are affected

46. ID TRIGGER ARGUMENT: AGENT(S)
c389 an emergency summit the leaders of both nations: ID=20:
CHAIN=20: PERS
c397 the two-hour closed meeting they: ID=24: CHAIN=20: PERS
c408 the summit Fujimori: ID=60: CHAIN=32: PERS
Hashimoto: ID=58:CHAIN=40:PERS
c409 the summit Fujimori: ID=60: CHAIN=32: PERS
Hashimoto: ID=58: CHAIN=40: PERS
c418 the summit rebels: ID=110: CHAIN=14: PERS
c432 the summit he: ID=170: CHAIN=40: PERS

47. Identity of sense
 There are cases where even though the strings are
the same we do not have identity of reference: at
least nine people and nine confirmed dead
 Hundred, at least 500 people, the first group of at
least 500 people, but probably more than that and
the 500
 It can be argued that events of INJURE, DIE and
DEFEND with such parameters are not
coreferential, but the ATTACK events that causes
them are.

48. at least nine people were killed and up to 37 wounded
i343 TRIGGER: wounded
AGENT: the FAZ (Zaire army): ID=53: CHAIN=53: ORG
VICTIM: up to 37: ID=66: CHAIN=66: PERSON
CAUSE: –
PLACE: Bukavu: ID=70: CHAIN=17: LOCATION
TIME: Monday: ID=69: CHAIN=7
there are nine confirmed dead and 37 wounded
i346 TRIGGER: wounded
AGENT: –
VICTIM: 37 wounded: ID=86: CHAIN=78: PERSON
CAUSE: –
PLACE: –
TIME: –

49. Missing slots
 Coreference between events can be established
even if many slots are not filled in:
 Peru’s Fujimori says hostage talks still young.
 ...the President said talks to free them were still in their
preliminary phase.
 ”We cannot predict how many more weeks these
discussions will take.”
 ”We are still at a preliminary stage in the conversations.”
 Fujimori said he hoped Nestor Cerpa would personally take
part in the talks when they resume on Monday at 11am.

50. Contact events
 Involve 2 or more parties
 The parties are usually introduced bit by bit
and event coreference is necessary to
establish all the participants
 Cross-document event coreference is
sometimes necessary collect all the
participants

51. Conclusions
 The guidelines should not be used directly and the
characteristics of the texts should be considered
 For automatic processing MUC-like may provide a good trade off
between the linguistic detail encoded and the difficulty of
annotation
 However, quite often this annotation is not enough for more
advanced processing
 Have a more refined notion of “identity”
Coreference is a scalar relation holding between two (or more)
linguistic expressions that refer to DEs considered to be at the
same granularity level relevant to the pragmatic purpose.
(Recasens, Hovy and Marti, forthcoming)

52. Thank you!

53. References
 van Deemter, Kees and Rodger Kibble, (1999). What is coreference and what
should coreference annotation be? In Amit Bagga, Breck Baldwin, and S Shelton
(eds.), Proceedings of ACL workshop on Coreference and Its Applications.
Maryland.
 Halliday, M. A. K., and Hasan, R. (1976).Cohesion in English. London: Longman.
 Hasler, L. and Orasan. C (2009). Do coreferential arguments make event mentions
coreferential? Proceedings of the 7th Discourse Anaphora and Anaphor Resolution
Colloquium (DAARC 2009), Goa, India, 5-6 November 2009, 151-163
 Hasler, L., Orasan, C. and Naumann, K. (2006) NPs for Events: Experiments in
coreference annotation. In Proceedings of the 5th Language Resources and
Evaluation Conference (LREC2006). Genoa, Italy, 24-26 May, 1167-1172
 Hirschman, L. (1997). MUC-7 coreference task definition. Version 3.0
 Mitkov, R. (2002): Anaphora Resolution. Longman
 Mitkov, R., Evans, R., Orasan, C., Barbu, C., Jones L. and Sotirova, V. (2000)
Coreference and anaphora: developing annotating tools, annotated resources and
annotation strategies Proceedings of the Discourse Anaphora and Anaphora
Resolution Colloquium (DAARC'2000)), 49-58. Lancaster, UK