pos tagging and syntactic parsing heng ji [email protected] september 10, 2014
TRANSCRIPT
Outline
• POS Tagging and HMM• Formal Grammars
• Context-free grammar• Grammars for English• Treebanks
• Parsing and CKY Algorithm
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What is Part-of-Speech (POS)
• Generally speaking, Word Classes (=POS) :• Verb, Noun, Adjective, Adverb, Article, …
• We can also include inflection:• Verbs: Tense, number, …• Nouns: Number, proper/common, …• Adjectives: comparative, superlative, …• …
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Parts of Speech• 8 (ish) traditional parts of speech
• Noun, verb, adjective, preposition, adverb, article, interjection, pronoun, conjunction, etc
• Called: parts-of-speech, lexical categories, word classes, morphological classes, lexical tags...
• Lots of debate within linguistics about the number, nature, and universality of these• We’ll completely ignore this debate.
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7 Traditional POS Categories
• N noun chair, bandwidth, pacing• V verb study, debate, munch• ADJ adj purple, tall, ridiculous• ADV adverb unfortunately, slowly,• P preposition of, by, to• PRO pronoun I, me, mine• DET determiner the, a, that, those
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POS Tagging• The process of assigning a part-of-speech or lexical
class marker to each word in a collection.WORD tag
the DETkoala Nput Vthe DETkeys Non Pthe DETtable N
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Penn TreeBank POS Tag Set
• Penn Treebank: hand-annotated corpus of Wall Street Journal, 1M words
• 46 tags• Some particularities:
• to /TO not disambiguated• Auxiliaries and verbs not distinguished
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Penn Treebank Tagset
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Why POS tagging is useful?• Speech synthesis:
• How to pronounce “lead”?• INsult inSULT• OBject obJECT• OVERflow overFLOW• DIScount disCOUNT• CONtent conTENT
• Stemming for information retrieval• Can search for “aardvarks” get “aardvark”
• Parsing and speech recognition and etc• Possessive pronouns (my, your, her) followed by nouns• Personal pronouns (I, you, he) likely to be followed by verbs• Need to know if a word is an N or V before you can parse
• Information extraction• Finding names, relations, etc.
• Machine Translation
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Open and Closed Classes• Closed class: a small fixed membership
• Prepositions: of, in, by, …• Auxiliaries: may, can, will had, been, …• Pronouns: I, you, she, mine, his, them, …• Usually function words (short common words which play a
role in grammar)• Open class: new ones can be created all the time
• English has 4: Nouns, Verbs, Adjectives, Adverbs• Many languages have these 4, but not all!
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Open Class Words• Nouns
• Proper nouns (Boulder, Granby, Eli Manning)• English capitalizes these.
• Common nouns (the rest). • Count nouns and mass nouns
• Count: have plurals, get counted: goat/goats, one goat, two goats• Mass: don’t get counted (snow, salt, communism) (*two snows)
• Adverbs: tend to modify things• Unfortunately, John walked home extremely slowly yesterday• Directional/locative adverbs (here,home, downhill)• Degree adverbs (extremely, very, somewhat)• Manner adverbs (slowly, slinkily, delicately)
• Verbs• In English, have morphological affixes (eat/eats/eaten)
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Closed Class WordsExamples:
• prepositions: on, under, over, …• particles: up, down, on, off, …• determiners: a, an, the, …• pronouns: she, who, I, ..• conjunctions: and, but, or, …• auxiliary verbs: can, may should, …• numerals: one, two, three, third, …
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Prepositions from CELEX
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English Particles
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Conjunctions
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POS TaggingChoosing a Tagset
• There are so many parts of speech, potential distinctions we can draw
• To do POS tagging, we need to choose a standard set of tags to work with
• Could pick very coarse tagsets• N, V, Adj, Adv.
• More commonly used set is finer grained, the “Penn TreeBank tagset”, 45 tags• PRP$, WRB, WP$, VBG
• Even more fine-grained tagsets exist
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Using the Penn Tagset• The/DT grand/JJ jury/NN commmented/VBD on/IN a/DT
number/NN of/IN other/JJ topics/NNS ./.• Prepositions and subordinating conjunctions marked IN
(“although/IN I/PRP..”)• Except the preposition/complementizer “to” is just marked
“TO”.
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POS Tagging• Words often have more than one POS: back
• The back door = JJ• On my back = NN• Win the voters back = RB• Promised to back the bill = VB
• The POS tagging problem is to determine the POS tag for a particular instance of a word.
These examples from Dekang Lin
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How Hard is POS Tagging? Measuring Ambiguity
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Current Performance
• How many tags are correct?• About 97% currently• But baseline is already 90%• Baseline algorithm:
• Tag every word with its most frequent tag• Tag unknown words as nouns
• How well do people do?
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Quick Test: Agreement?
• the students went to class• plays well with others• fruit flies like a banana
DT: the, this, thatNN: nounVB: verbP: prepostionADV: adverb
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Quick Test
• the students went to class
DT NN VB P NN• plays well with others
VB ADV P NN
NN NN P DT• fruit flies like a banana
NN NN VB DT NN
NN VB P DT NN
NN NN P DT NN
NN VB VB DT NN
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How to do it? History
1960 1970 1980 1990 2000
Brown Corpus Created (EN-US)1 Million Words
Brown Corpus Tagged
HMM Tagging (CLAWS)93%-95%
Greene and RubinRule Based - 70%
LOB Corpus Created (EN-UK)1 Million Words
DeRose/ChurchEfficient HMMSparse Data
95%+
British National Corpus
(tagged by CLAWS)
POS Tagging separated from
other NLP
Transformation Based Tagging
(Eric Brill)Rule Based – 95%+
Tree-Based Statistics (Helmut Shmid)
Rule Based – 96%+
Neural Network 96%+
Trigram Tagger(Kempe)
96%+
Combined Methods98%+
Penn Treebank Corpus
(WSJ, 4.5M)
LOB Corpus Tagged
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Two Methods for POS Tagging
1. Rule-based tagging• (ENGTWOL)
2. Stochastic1. Probabilistic sequence models
• HMM (Hidden Markov Model) tagging• MEMMs (Maximum Entropy Markov Models)
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Rule-Based Tagging• Start with a dictionary• Assign all possible tags to words from the dictionary• Write rules by hand to selectively remove tags• Leaving the correct tag for each word.
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Rule-based taggers
• Early POS taggers all hand-coded• Most of these (Harris, 1962; Greene and Rubin, 1971)
and the best of the recent ones, ENGTWOL (Voutilainen, 1995) based on a two-stage architecture• Stage 1: look up word in lexicon to give list of potential POSs• Stage 2: Apply rules which certify or disallow tag sequences
• Rules originally handwritten; more recently Machine Learning methods can be used
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Start With a Dictionary• she: PRP• promised: VBN,VBD• to TO• back: VB, JJ, RB, NN• the: DT• bill: NN, VB
• Etc… for the ~100,000 words of English with more than 1 tag
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Assign Every Possible Tag
NN
RB
VBN JJ VB
PRP VBD TO VB DT NN
She promised to back the bill
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Write Rules to Eliminate Tags
Eliminate VBN if VBD is an option when VBN|VBD follows “<start> PRP”
NN
RB
JJ VB
PRP VBD TO VB DT NN
She promised to back the bill
VBN
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POS tagging
The involvement of ion channels in B and T lymphocyte activation is DT NN IN NN NNS IN NN CC NN NN NN VBZ supported by many reports of changes in ion fluxes and membrane VBN IN JJ NNS IN NNS IN NN NNS CC NN…………………………………………………………………………………….…………………………………………………………………………………….
Machine Learning Algorithm
training
We demonstrate that …
Unseen text
We demonstratePRP VBPthat … IN
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Goal of POS Tagging
We want the best set of tags for a sequence of words (a sentence)
W — a sequence of wordsT — a sequence of tags
)|(maxarg^
WTPTT
Example: P((NN NN P DET ADJ NN) | (heat oil in a large pot))
OurGoal
OurGoal
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But, the Sparse Data Problem …
• Rich Models often require vast amounts of data• Count up instances of the string "heat oil in a large pot" in
the training corpus, and pick the most common tag assignment to the string..
• Too many possible combinations
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POS Tagging as Sequence Classification
• We are given a sentence (an “observation” or “sequence of observations”)• Secretariat is expected to race tomorrow
• What is the best sequence of tags that corresponds to this sequence of observations?
• Probabilistic view:• Consider all possible sequences of tags• Out of this universe of sequences, choose the tag sequence which is
most probable given the observation sequence of n words w1…wn.
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Getting to HMMs• We want, out of all sequences of n tags t1…tn the single tag sequence
such that P(t1…tn|w1…wn) is highest.
• Hat ^ means “our estimate of the best one”• Argmaxx f(x) means “the x such that f(x) is maximized”
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Getting to HMMs• This equation is guaranteed to give us the best tag
sequence
• But how to make it operational? How to compute this value?
• Intuition of Bayesian classification:• Use Bayes rule to transform this equation into a set of other
probabilities that are easier to compute
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Reminder: ApplyBayes’ Theorem (1763)
)(
)()|()|(
WP
TPTWPWTP
posteriorposterior priorprior
likelihoodlikelihood
marginal likelihoodmarginal likelihood
Reverend Thomas Bayes — Presbyterian minister (1702-1761)Reverend Thomas Bayes — Presbyterian minister (1702-1761)
Our Goal: To maximize it!
Our Goal: To maximize it!
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How to Count
P(W|T) and P(T) can be counted from a large
hand-tagged corpus; and smooth them to get rid of the zeroes
)|(maxarg^
WTPTT
)(
)()|(maxarg
WP
TPTWP
T
)()|(maxarg TPTWPT
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Count P(W|T) and P(T)
Assume each word in the sequence depends only on its corresponding tag:
n
i
ii twPTWP1
)|()|(
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Make a Markov assumption and use N-grams over tags ...
P(T) is a product of the probability of N-grams that make it up
Make a Markov assumption and use N-grams over tags ...
P(T) is a product of the probability of N-grams that make it up
Count P(T)
),...,|(...)|()|()(
),...,(
11213121
1
nn
n
tttPtttPttPtP
ttP historyhistory
n
i
iin ttPtPttP2
111 )|()(),...,(
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Part-of-speech tagging with Hidden Markov Models
n
iiiii
nnn
n
nnnnn
ttPtwP
ttPttwwP
wwP
ttPttwwPwwttP
11
111
1
11111
||
......|...
...
......|......|...
wordstags
output probability transition probability
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Analyzing
Fish sleep.
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A Simple POS HMM
start noun verb end0.8
0.2
0.80.7
0.1
0.2
0.10.1
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Word Emission ProbabilitiesP ( word | state )• A two-word language: “fish” and “sleep”• Suppose in our training corpus,
• “fish” appears 8 times as a noun and 5 times as a verb• “sleep” appears twice as a noun and 5 times as a verb
• Emission probabilities:• Noun
• P(fish | noun) : 0.8• P(sleep | noun) : 0.2
• Verb• P(fish | verb) : 0.5• P(sleep | verb) : 0.5
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Viterbi Probabilities
0 1 2 3
start
verb
noun
end
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0 1 2 3
start 1
verb 0
noun 0
end 0
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
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0 1 2 3
start 1 0
verb 0 .2 * .5
noun 0 .8 * .8
end 0 0
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 1: fish
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0 1 2 3
start 1 0
verb 0 .1
noun 0 .64
end 0 0
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 1: fish
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0 1 2 3
start 1 0 0
verb 0 .1 .1*.1*.5
noun 0 .64 .1*.2*.2
end 0 0 -
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 2: sleep
(if ‘fish’ is verb)
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0 1 2 3
start 1 0 0
verb 0 .1 .005
noun 0 .64 .004
end 0 0 -
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 2: sleep
(if ‘fish’ is verb)
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0 1 2 3
start 1 0 0
verb 0 .1 .005.64*.8*.5
noun 0 .64 .004.64*.1*.2
end 0 0 -
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 2: sleep
(if ‘fish’ is a noun)
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0 1 2 3
start 1 0 0
verb 0 .1 .005.256
noun 0 .64 .004.0128
end 0 0 -
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 2: sleep
(if ‘fish’ is a noun)
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0 1 2 3
start 1 0 0
verb 0 .1 .005.256
noun 0 .64 .004.0128
end 0 0 -
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 2: sleeptake maximum,set back pointers
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0 1 2 3
start 1 0 0
verb 0 .1 .256
noun 0 .64 .0128
end 0 0 -
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 2: sleeptake maximum,set back pointers
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0 1 2 3
start 1 0 0 0
verb 0 .1 .256 -
noun 0 .64 .0128 -
end 0 0 - .256*.7.0128*.1
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 3: end
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0 1 2 3
start 1 0 0 0
verb 0 .1 .256 -
noun 0 .64 .0128 -
end 0 0 - .256*.7.0128*.1
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Token 3: endtake maximum,set back pointers
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0 1 2 3
start 1 0 0 0
verb 0 .1 .256 -
noun 0 .64 .0128 -
end 0 0 - .256*.7
start noun verb end0.8
0.2
0.8 0.7
0.1
0.2
0.1 0.1
Decode:fish = nounsleep = verb
Markov Chain for a Simple Name Tagger
STARTEND
PER
X
0.30.2
0.2
0.2 0.3
0.6
0.5
George:0.3
W.:0.3
W.:0.3
discussed:0.7
$:1.0
LOC
0.5
0.2
0.1
0.30.3 0.1
0.2
Bush:0.3
Iraq:0.1
George:0.2
Iraq:0.8
Transition Probability
Emission Probability
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Exercise• Tag names in the following sentence:• George. W. Bush discussed Iraq.
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POS taggers• Brill’s tagger
• http://www.cs.jhu.edu/~brill/• TnT tagger
• http://www.coli.uni-saarland.de/~thorsten/tnt/• Stanford tagger
• http://nlp.stanford.edu/software/tagger.shtml• SVMTool
• http://www.lsi.upc.es/~nlp/SVMTool/• GENIA tagger
• http://www-tsujii.is.s.u-tokyo.ac.jp/GENIA/tagger/• More complete list at: http://www-nlp.stanford.edu/links/statnlp.html#Taggers
Outline
• Query Expansion and Relevance Feedback• POS Tagging and HMM• Formal Grammars
• Context-free grammar• Grammars for English• Treebanks
• Parsing and CKY Algorithm
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Syntax• By grammar, or syntax, we have in mind the kind of
implicit knowledge of your native language that you had mastered by the time you were 3 years old without explicit instruction
• Not the kind of stuff you were later taught in “grammar” school
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Syntax• Why should you care?• Grammars (and parsing) are key components in many
applications• Grammar checkers• Dialogue management• Question answering • Information extraction• Machine translation
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Syntax• Key notions that we’ll cover
• Constituency• Grammatical relations and Dependency
• Heads
• Key formalism• Context-free grammars
• Resources• Treebanks
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Constituency• The basic idea here is that groups of words within
utterances can be shown to act as single units.• And in a given language, these units form coherent
classes that can be be shown to behave in similar ways• With respect to their internal structure• And with respect to other units in the language
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Constituency• Internal structure
• We can describe an internal structure to the class (might have to use disjunctions of somewhat unlike sub-classes to do this).
• External behavior• For example, we can say that noun phrases can come before
verbs
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Constituency• For example, it makes sense to the say that the following
are all noun phrases in English...
• Why? One piece of evidence is that they can all precede verbs.• This is external evidence
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Grammars and Constituency• Of course, there’s nothing easy or obvious about how we
come up with right set of constituents and the rules that govern how they combine...
• That’s why there are so many different theories of grammar and competing analyses of the same data.
• The approach to grammar, and the analyses, adopted here are very generic (and don’t correspond to any modern linguistic theory of grammar).
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Context-Free Grammars• Context-free grammars (CFGs)
• Also known as• Phrase structure grammars• Backus-Naur form
• Consist of• Rules • Terminals• Non-terminals
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Context-Free Grammars• Terminals
• We’ll take these to be words (for now)
• Non-Terminals• The constituents in a language
• Like noun phrase, verb phrase and sentence
• Rules• Rules are equations that consist of a single non-terminal on the left
and any number of terminals and non-terminals on the right.
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Some NP Rules• Here are some rules for our noun phrases
• Together, these describe two kinds of NPs.• One that consists of a determiner followed by a nominal• And another that says that proper names are NPs.• The third rule illustrates two things
• An explicit disjunction• Two kinds of nominals
• A recursive definition• Same non-terminal on the right and left-side of the rule
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L0 Grammar
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Derivations• A derivation is a sequence
of rules applied to a string that accounts for that string• Covers all the elements in the
string• Covers only the elements in
the string
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Definition• More formally, a CFG consists of
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Parsing• Parsing is the process of taking a string and a grammar
and returning a (multiple?) parse tree(s) for that string• It is completely analogous to running a finite-state
transducer with a tape• It’s just more powerful
• Remember this means that there are languages we can capture with CFGs that we can’t capture with finite-state methods
• More on this when we get to Ch. 13.
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An English Grammar Fragment• Sentences• Noun phrases
• Agreement
• Verb phrases• Subcategorization
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Sentence Types• Declaratives: A plane left.
S NP VP
• Imperatives: Leave!
S VP
• Yes-No Questions: Did the plane leave?
S Aux NP VP
• WH Questions: When did the plane leave?
S WH-NP Aux NP VP
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Noun Phrases• Let’s consider the following rule in more detail...
NP Det Nominal• Most of the complexity of English noun phrases is hidden
in this rule.• Consider the derivation for the following example
• All the morning flights from Denver to Tampa leaving before 10
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Noun Phrases
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NP Structure• Clearly this NP is really about flights. That’s the central
criticial noun in this NP. Let’s call that the head.• We can dissect this kind of NP into the stuff that can
come before the head, and the stuff that can come after it.
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Determiners• Noun phrases can start with determiners...• Determiners can be
• Simple lexical items: the, this, a, an, etc.• A car
• Or simple possessives• John’s car
• Or complex recursive versions of that• John’s sister’s husband’s son’s car
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Nominals• Contains the head and any pre- and post- modifiers of the
head.• Pre-
• Quantifiers, cardinals, ordinals...• Three cars
• Adjectives and Aps• large cars
• Ordering constraints• Three large cars• ?large three cars
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Postmodifiers• Three kinds
• Prepositional phrases• From Seattle
• Non-finite clauses• Arriving before noon
• Relative clauses• That serve breakfast
• Same general (recursive) rule to handle these• Nominal Nominal PP• Nominal Nominal GerundVP• Nominal Nominal RelClause
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Agreement• By agreement, we have in mind constraints that hold
among various constituents that take part in a rule or set of rules
• For example, in English, determiners and the head nouns in NPs have to agree in their number.
This flight
Those flights
*This flights
*Those flight
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The Point• CFGs appear to be just about what we need to account
for a lot of basic syntactic structure in English.• But there are problems
• That can be dealt with adequately, although not elegantly, by staying within the CFG framework.
• There are simpler, more elegant, solutions that take us out of the CFG framework (beyond its formal power)• LFG, HPSG, Construction grammar, XTAG, etc.• Chapter 15 explores the unification approach in more detail
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Treebanks• Treebanks are corpora in which each sentence has been
paired with a parse tree (presumably the right one).• These are generally created
• By first parsing the collection with an automatic parser• And then having human annotators correct each parse as
necessary.• This generally requires detailed annotation guidelines that
provide a POS tagset, a grammar and instructions for how to deal with particular grammatical constructions.
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Penn Treebank• Penn TreeBank is a widely used treebank.
Most well known is the Wall Street Journal section of the Penn TreeBank.
1 M words from the 1987-1989 Wall Street Journal.
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Treebank Grammars• Treebanks implicitly define a grammar for the language
covered in the treebank.• Simply take the local rules that make up the sub-trees in
all the trees in the collection and you have a grammar.• Not complete, but if you have decent size corpus, you’ll
have a grammar with decent coverage.
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Treebank Grammars• Such grammars tend to be very flat due to the fact that
they tend to avoid recursion.• To ease the annotators burden
• For example, the Penn Treebank has 4500 different rules for VPs. Among them...
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Heads in Trees• Finding heads in treebank trees is a task that arises
frequently in many applications.• Particularly important in statistical parsing
• We can visualize this task by annotating the nodes of a parse tree with the heads of each corresponding node.
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Lexically Decorated Tree
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Head Finding• The standard way to do head finding is to use a simple set
of tree traversal rules specific to each non-terminal in the grammar.
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Noun Phrases
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Treebank Uses• Treebanks (and headfinding) are particularly critical to the
development of statistical parsers• Chapter 14
• Also valuable to Corpus Linguistics • Investigating the empirical details of various constructions in a
given language
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Summary• Context-free grammars can be used to model various
facts about the syntax of a language.• When paired with parsers, such grammars consititute a
critical component in many applications.• Constituency is a key phenomena easily captured with
CFG rules.• But agreement and subcategorization do pose significant
problems• Treebanks pair sentences in corpus with their
corresponding trees.
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For Now• Assume…
• You have all the words already in some buffer• The input isn’t POS tagged• We won’t worry about morphological analysis• All the words are known
• These are all problematic in various ways, and would have to be addressed in real applications.
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Top-Down Search• Since we’re trying to find trees rooted with an S
(Sentences), why not start with the rules that give us an S.
• Then we can work our way down from there to the words.
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Top Down Space
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Bottom-Up Parsing• Of course, we also want trees that cover the input words.
So we might also start with trees that link up with the words in the right way.
• Then work your way up from there to larger and larger trees.
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Bottom-Up Search
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Bottom-Up Search
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Bottom-Up Search
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Bottom-Up Search
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Bottom-Up Search
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Top-Down and Bottom-Up• Top-down
• Only searches for trees that can be answers (i.e. S’s)• But also suggests trees that are not consistent with any of the
words
• Bottom-up• Only forms trees consistent with the words• But suggests trees that make no sense globally
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Control• Of course, in both cases we left out how to keep track of
the search space and how to make choices• Which node to try to expand next• Which grammar rule to use to expand a node
• One approach is called backtracking.• Make a choice, if it works out then fine• If not then back up and make a different choice
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Problems• Even with the best filtering, backtracking methods are
doomed because of two inter-related problems• Ambiguity• Shared subproblems
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Ambiguity
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Shared Sub-Problems• No matter what kind of search (top-down or bottom-up or
mixed) that we choose.• We don’t want to redo work we’ve already done.• Unfortunately, naïve backtracking will lead to duplicated work.
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Shared Sub-Problems
• Consider• A flight from Indianapolis to Houston on TWA
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Shared Sub-Problems• Assume a top-down parse making choices among the
various Nominal rules.• In particular, between these two
• Nominal -> Noun• Nominal -> Nominal PP
• Statically choosing the rules in this order leads to the following bad results...
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Shared Sub-Problems
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Shared Sub-Problems
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Shared Sub-Problems
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Shared Sub-Problems
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Dynamic Programming• DP search methods fill tables with partial results and
thereby• Avoid doing avoidable repeated work• Solve exponential problems in polynomial time (well, no not
really)• Efficiently store ambiguous structures with shared sub-parts.
• We’ll cover the CKY algorithm
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CKY Parsing
• First we’ll limit our grammar to epsilon-free, binary rules (more later)
• Consider the rule A BC• If there is an A somewhere in the input then there must be
a B followed by a C in the input.• If the A spans from i to j in the input then there must be
some k st. i<k<j• Ie. The B splits from the C someplace.
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Problem
• What if your grammar isn’t binary?• As in the case of the TreeBank grammar?
• Convert it to binary… any arbitrary CFG can be rewritten into Chomsky-Normal Form automatically.
• What does this mean?• The resulting grammar accepts (and rejects) the
same set of strings as the original grammar.• But the resulting derivations (trees) are different.
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Problem• More specifically, we want our rules to be of the form
A B C
Or
A w
That is, rules can expand to either 2 non-terminals or to a single terminal.
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Binarization Intuition
• Eliminate chains of unit productions.• Introduce new intermediate non-terminals into the
grammar that distribute rules with length > 2 over several rules. • So… S A B C turns into S X C andX A BWhere X is a symbol that doesn’t occur anywhere else in the the grammar.
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Sample L1 Grammar
the
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CNF Conversion
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CKY• So let’s build a table so that an A spanning from i to j in
the input is placed in cell [i,j] in the table.• So a non-terminal spanning an entire string will sit in cell
[0, n]• Hopefully an S
• If we build the table bottom-up, we’ll know that the parts of the A must go from i to k and from k to j, for some k.
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CKY• Meaning that for a rule like A B C we should look for a B
in [i,k] and a C in [k,j].• In other words, if we think there might be an A spanning i,j
in the input… AND
A B C is a rule in the grammar THEN• There must be a B in [i,k] and a C in [k,j] for some i<k<j
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CKY• So to fill the table loop over the cell[i,j] values in some
systematic way• What constraint should we put on that systematic search?
• For each cell, loop over the appropriate k values to search for things to add.
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Example
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Example
Filling column 5
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Example
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Example
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Example
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Example
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To formalize it: CKY Algorithm
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Exercises
• Try to parse the following sentence:• I prefer meal on flight.
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Take-home Messages• Context-free grammars can be used to model various
facts about the syntax of a language.• When paired with parsers, such grammars consititute a
critical component in many applications.• Constituency is a key phenomena easily captured with
CFG rules.• But agreement and subcategorization do pose significant
problems• CKY is a bottom-up dynamic programming algorithm• We can convert CFG rules into CNF forms