microplanning (sentence planning) part 1 kees van deemter
TRANSCRIPT
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Microplanning(Sentence planning)
Part 1
Kees van Deemter
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Natural Language Generation
• Taking some computer-readable gibberish
• “Translating” it into proper English
• Applications include– dialogue/chat systems– on-line help– summarisation, – document authoring
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NLG Tasks (as explained by Anja):
1. Content determination: decide what to say; construct set of messages
2. Discourse planning: ordering, structuring concepts; rhetorical relationships
3. Sentence aggregation: divide content into sentences; construct sentence plans
4. Lexicalisation: map concepts and relations to lexemes (= words)
5. Referring expression generation: decide how to refer to objects
6. Linguistic realisation: put it all together in acceptable words and sentences
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Modular structure of NLG systems (in theory!):
Content determination
Discourse planning
Sentence aggregation
Realisation
Lexicalisation
Referring expressions
TEXT PLANNER
REALISER
SENTENCE PLANNER/MICROPLANNER
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Last week: Input to realisation
message-id: msg02
relation: C_DEPARTURE
departing-entity: C_CALEDON-EXPRESS
args: departure-location: C_ABERDEEN
departure-time: C_1000
departure-platform: C_7
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Microplanning 1:Aggregation
• Distributing information over different sentences. Example:
a. The Caledonian express departs Aberdeen at 10:00, from platform 7
b. The Caledonian express departs Aberdeen at 10:00. The Caledonia express departs from platform 7
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Microplanning 2: GRE
GRE = Generation of Referring Expressions
Explaining which objects you’re talking about
a. The Caledonian express departs Aberdeen at 10:00, from platform 7
b. The Caledonian express departs -- at 10:00. The train departs from this platform
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Microplanning 3: lexical choice
Using different words for the same concept
a. The Caledonian express departs Aberdeen at ten o’clock, from platform 7
b. The Caledonian express departs Aberdeen at ten. The Caledonia express leaves from platform 7
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In practice: tasks can be performed in different order
• Example: aggregation can be performedon messages:
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message-id: msg02
relation: C_DEPARTURE_1
departing-entity: C_CALEDON-EXPRESS
args: departure-location: C_ABERDEEN
departure-time: C_1000
message-id: msg03
relation: C_DEPARTURE_2
args: departure-entity: C_CALEDON-EXPRESS
departure-platform: C_7
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• Aggregation can also be performed later:
[The Caledonian express] departs Aberdeen [at 10:00] [from platform 7]
===> [The Caledonian express] departs Aberdeen
[at 10:00]. [The Caledonia express] departs [from platform 7]
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Let’s focus on GRE, but ...
• A little detour: NLG systems do not always work as you’ve been told
• Some practically deployed systems combine “canned text” with NLG
• One possibility: system has a library of language “templates”, with gaps that need to be filled. E.g.,
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[TRAIN] departs [TOWN] at [TIME]
[TRAIN] departs [TOWN] from [PLATFORM]
We apologise for the fact that [TRAIN] is delayed by [AMOUNT]
Gap filling: using canned text or GRE.
Question: which of the other tasks are still relevant?
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Let’s move on to GRE
• Why/when is GRE useful?
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1. The referent has a familiar name, but it’s not unique, e.g., ‘John Smith’
2. The referent has no familiar name: trains, furniture, trees, atomic particles, …
( Databases use keys, e.g.,
‘Smith$73527$’, ‘TRAIN-3821’ )
3. Similar: sets of objects
4. NL is too economical to have namesfor everything
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Last week: Input to realisation
message-id: msg02
relation: C_DEPARTURE
departing-entity: C_CALEDON-EXPRESS
args: departure-location: C_ABERDEEN
departure-time: C_1000
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Last week: Input to realisation
message-id: msg02
relation: C_DEPARTURE
departing-entity: C_CALEDON-EXPRESS
args: departure-location: C_ABERDEEN
departure-time: C_1000
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This week: more realistic input
message-id: msg02
relation: C_DEPARTURE
departing-entity: C_34435
args: departure-location: .....
departure-time: .....
“the caledonian (express)”,
“the Aberdeen-Glasgow express’’
“the blue train on your left” , “the train”
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• Communication is about saying the truth ...
• but that’s not all there is to it
• Paul Grice (around 1970): principles of rational, cooperative communication
• GRE, it a good case study. (R.Dale and E.Reiter, Cognitive Science, 1995)
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Grice: maxims of conversation
• Quality: only say what you know to be true
• Quantity: give enough but not too much information
• Relevance: be relevant
• Manner: be clear and brief
(There is overlap between these four)
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Maxims are two-edged sword:
1. They say how one should normally speak/write. Example:
“Yes, there’s a gasoline station around the corner” (when it’s no longer operational)
quality: yes, it’s truequantity: probably yesrelevance: no, not relevant to hearer’s intentionsmanner: it’s brief, clear, etc.
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Maxims are two-edged sword:
2. They can also be exploited. Example:
Asked to write academic reference: “Kees always came to my lectures and he’s a nice guy”
quality: yes, it’s true (let’s assume)
quantity: No -- How about academic achievements?
relevance: yes
manner: yes
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Application to GRE
Dale & Reiter: best description of an object fulfils the Gricean maxims. E.g.,
• (Quality:) list properties truthfully• (Quantity:) use properties that allow identification –
without containing more info• (Relevance:) use properties that are of interest in the
situation• (Manner:) be brief
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D&R’s expectation:
• Violation of a maxim leads to implicatures.
• For example,– [Quantity] ‘the pitbull’ (when there is
only one dog).– [Manner] ‘Get the cordless drill that’s
in the toolbox’ (Appelt).
• There’s just one problem: …
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people don’t always speak this way
For example,– [Manner] ‘the red chair’ (when there is
only one red object in the domain).
– [Manner/Quantity] ‘I broke my arm’ (when I have two).
General: empirical work shows much redundancy
Similar for other maxims, e.g.,– [Quality] ‘the man with the martini’ (Donellan)
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Example Situation
a, £100 b, £150
c, £100 d, £150 e, £?Swedish Italian
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Formalized in a KB
• Type: furniture (abcde), desk (ab), chair (cde)
• Origin: Sweden (ac), Italy (bde)
• Colours: dark (ade), light (bc), grey (a)
• Price: 100 (ac), 150 (bd) , 250 ({})
• Contains: wood ({}), metal (abcde), cotton(d)
Assumption: all this is shared knowledge.
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Game
1. Describe object a.
2. Describe object e.
3. Describe object d.
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Game
1. Describe object a: {desk,sweden}, {grey}
2. Describe object e: no solution
3. Describe object d: {Italy, 150}
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Violations of …• Manner:
* ‘The £100 grey Swedish desk which is made of metal’
(Description of a)
• Relevance: ‘The cotton chair is a fire hazard?
?Then why not buy the Swedish chair?’ (Descriptions of d and c respectively)
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• In fact, there is a second problem with Quantity/Manner. Consider the following formalization:
Full Brevity: Never use more than the minimal number of properties required for identification (Dale 1989)
An algorithm:
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Dale 1989:
1. Check whether 1 property is enough
2. Check whether 2 properties is enough
….
Etc., until
success {minimal description is generated} or
failure {no description is possible}
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Problem: exponential complexity
• Worst-case, this algorithm would have to inspect all combinations of properties. n properties combinations.
• Recall: one grain of rice on square one; twice as many on any subsequent square.
• Some algorithms may be faster, but …
• Theoretical result: algorithm must be exponential in the number of properties.
n2
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• D&R conclude that Full Brevity cannot be achieved in practice.
• They designed an algorithm that only approximates Full Brevity:
the Incremental Algorithm.
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Incremental Algorithm (informal):
• Properties are considered in a fixed order:
P =
• A property is included if it is ‘useful’:
true of target; false of some distractors
• Stop when done; so earlier properties have a greater chance of being included. (E.g., a perceptually salient property)
• Therefore called preference order.
nPPPP ,...,,, 321
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• r = individual to be described
• P = list of properties, in preference order
• P is a property
• L= properties in generated description
(Recall: we’re not worried about realization today)
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FailureReturn
LReturn then {r}C If
]][[C:C
}{L:L
do then ]][[ C &]][[r If
:do P allFor
Domain:C
Φ:L
P
P
PP
P
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Back to the KB
• Type: furniture (abcde), desk (ab), chair (cde)
• Origin: Sweden (ac), Italy (bde)
• Colours: dark (ade), light (bc), grey (a)
• Price: 100 (ac), 150 (bd) , 250 ({})
• Contains: wood ({}), metal (abcde), cotton(d)
Assumption: all this is shared knowledge.
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Back to our game
1. Describe object a.
2. Describe object e.
3. Describe object d.
Can you see room for improvement?