Natural Language Processing

Transcription

1 Natural Language Processing Part-of-Speech Tagging Joakim Nivre Uppsala University Department of Linguistics and Philology Natural Language Processing 1(13)

2 Parts of Speech I Basic grammatical categories used since antiquity: 1. Noun 2. Verb 3. Adjective 4. Adverb 5. Preposition 6. Pronoun 7. Conjunction 8. Interjection I Lots of debate in linguistics about their nature and universality I Nevertheless very robust and useful for NLP Natural Language Processing 2(13)

3 Part-of-Speech Tagging I Assign a part-of-speech tag to every word of a sentence Word Tag Holmes PROPN put VERB the DET keys NOUN on ADP the DET table NOUN. PUNCT Natural Language Processing 3(13)

4 Why is PoS tagging useful? I First step in a vast number of practical tasks 1. Text-to-speech how to pronounce lead or insult? 2. Parsing need to know if a word is NOUN or VERB 3. Information extraction finding names, relations, etc. I Used as a backoff model for word tokens (sparse data) Natural Language Processing 4(13)

5 Why is PoS tagging hard? I Lexical ambiguity: 1. Prince is expected to race/verb tomorrow 2. People wonder about the race/noun for outer space I Unknown words: 1. The rural Babbitt who bloviates about progress and growth Natural Language Processing 5(13)

6 How is it done? I Lexical information (the word itself) I Known words can be looked up in a lexicon listing possible tags for each word I Unknown words can be analyzed with respect to affixes, capitalization, special symbols, etc. I Contextual information (surrounding words) I A language model can rank tags in context I Many different models and techniques (more later) Natural Language Processing 6(13)

7 Quiz 1 I Consider the following incomplete sentence: She sent a letter to the... I Which parts of speech are likely to occur next? 1. ADJ 2. NOUN 3. VERB Natural Language Processing 7(13)

8 Tag Sets I There are many potential distinctions we can draw I Tag sets range from coarse-grained to fine-grained 1. Universal Dependencies: 17 tags 2. Penn Treebank, English: 45 tags 3. SUC, Swedish: 25 tags + features 150 tags I Choice of tag set may depend on application Natural Language Processing 8(13)

9 Universal Dependencies (UD) Open class words Closed class words Other ADJ adjective ADP preposition/postposition PUNCT punctuation ADV adverb AUX auxiliary verb SYM symbol INTJ interjection CONJ coordinating conjunction X unspecified NOUN noun DET determiner PROPN proper noun NUM numeral VERB verb PART particle PRON pronoun SCONJ subordinating conjunction Natural Language Processing 9(13)

10 Penn Treebank Natural Language Processing 10(13)

11 How hard is PoS tagging? Natural Language Processing 11(13)

12 Evaluation I Evaluation against a manually annotated gold standard I Evaluation metrics: I Accuracy = percentage of correctly tagged tokens I Separate results for ambiguous and/or unknown words I State of the art: I I I 96 98% for English news text What about Turkish? What about Twitter? Natural Language Processing 12(13)

13 Quiz 2 I Consider the following tagging: She/PRON won/verb the/det race/verb I What accuracy score would you give it? % 2. 75% 3. 50% Natural Language Processing 13(13)

14 Natural Language Processing Tagging Methods Joakim Nivre Uppsala University Department of Linguistics and Philology Natural Language Processing 1(14)

15 Part-of-Speech Tagging I Task: I Assign a part-of-speech tag to every word of a sentence I Useful tools and techniques: I Lexicon mapping words to possible tags I Linguistic rules for disambiguation in context I Statistical models of tags and words in context I Heuristics for handling unknown words I In this lecture: I Transformation-based tagging a rule-based approach I HMM tagging a statistical approach Natural Language Processing 2(14)

16 Transformation-Based Tagging I Assign each word its most frequent tag Prince is expected to race/noun tomorrow People wonder about the race/noun for outer space Prince is expected to run/verb tomorrow People wonder about the run/verb for charity Natural Language Processing 3(14)

17 Transformation-Based Tagging I Assign each word its most frequent tag Prince is expected to race/noun tomorrow People wonder about the race/noun for outer space Prince is expected to run/verb tomorrow People wonder about the run/verb for charity I Use a sequence of rules to refine the tagging 1. NOUN! VERB if preceding word is to 2. VERB! NOUN if preceding word is the Natural Language Processing 3(14)

18 Transformation-Based Tagging I Assign each word its most frequent tag Prince is expected to race/noun tomorrow People wonder about the race/noun for outer space Prince is expected to run/verb tomorrow People wonder about the run/verb for charity I Use a sequence of rules to refine the tagging 1. NOUN! VERB if preceding word is to 2. VERB! NOUN if preceding word is the Prince is expected to race/verb tomorrow People wonder about the race/noun for outer space Prince is expected to run/verb tomorrow People wonder about the run/noun for charity Natural Language Processing 3(14)

19 Transformation-Based Tagging I Learning a set of rules from a tagged corpus: 1. Define a set of rule templates 2. Assign every word its most frequent tag 3. Repeat until no further improvement: 3.1 Apply every rule to the current tagged corpus by itself 3.2 Add the best rule R to the sequence of rules 3.3 Transform the current tagged corpus using R Natural Language Processing 4(14)

20 Transformation-Based Tagging I Learning a set of rules from a tagged corpus: 1. Define a set of rule templates 2. Assign every word its most frequent tag 3. Repeat until no further improvement: 3.1 Apply every rule to the current tagged corpus by itself 3.2 Add the best rule R to the sequence of rules 3.3 Transform the current tagged corpus using R I Using the rules to tag a new text: 1. Assign every word its most frequent tag 2. For every rule R 1,...,R n in the learned sequence: 2.1 Transform the current tagged corpus using R i Natural Language Processing 4(14)

21 Quiz 1 I Consider the following initial taggings of the word light: light/verb the/det candle/noun see/verb the/det light/verb carry/verb the/det light/verb suitcase/noun... I And suppose we apply the following two rules in sequence: 1. VERB! NOUN if preceding word is DET 2. NOUN! ADJ if preceding word is DET and following word is NOUN I Which of the following statements are true? 1. All three occurrences are correctly tagged in the end 2. There is at least one error in the end tagging 3. Removing the second rule gives one more error in the end 4. Switching the order of the rules has no impact on the end result Natural Language Processing 5(14)

22 Statistical Tagging I Basic ideas: I Build a statistical model of words and their tags I Estimate model parameters from (tagged) corpus data I Use the model to assign the most probable tags to words I Example: I Part-of-speech tagging using Hidden Markov Models (HMM) Natural Language Processing 6(14)

23 Hidden Markov Models I Markov models are probabilistic sequence models used for problems such as: 1. Speech recognition 2. Spell checking 3. Part-of-speech tagging 4. Named entity recognition I Given a word sequence w 1,...,w n,wewantto find the most probable tag sequence t 1,...,t n : argmax t 1,...,t n P(t 1,...,t n w 1,...,w n ) Natural Language Processing 7(14)

24 Model Construction I Bayesian inversion: P(t 1,...,t n w 1,...,w n )= P(t 1,...,t n )P(w 1,...,w n t 1,...,t n ) P(w 1,...,w n ) I Submodels: 1. Prior: P(t 1,...,t n ) 2. Likelihood: P(w 1,...,w n t 1,...,t n ) 3. Marginal: P(w 1,...,w n ) canbeignoredinargmaxsearch Natural Language Processing 8(14)

25 Markov Assumptions I Context model (prior) P(t 1,...,t n )= I Lexical model (likelihood) ny P(t i t i k...,t i 1 ) i=1 ny P(w 1,...,w n t 1,...,t n )= P(w i t i ) i=1 Natural Language Processing 9(14)

26 Model Parameters I Contextual probabilities P(t i t i k,...,t i 1 ) I Lexical probabilities P(w i t i ) I We can estimate these probabilities from a tagged corpus: ˆP MLE (w i t i )= f (w i, t i ) f (t i ) ˆP MLE (t i t i k,...,t i 1 )= f (t i k,...,t i 1, t i ) f (t i k,...,t i 1 ) Natural Language Processing 10(14)

27 Computing Probabilities I The probability of a tagging: P(t 1,...,t n, w 1,...,w n )= ny P(t i t i k,...,t i 1 )P(w i t i ) i=1 I Finding the most probable tagging: argmax t 1,...,t n ny P(t i t i k,...,t i 1 )P(w i t i ) i=1 I This requires an efficient algorithm (more later) Natural Language Processing 11(14)

28 Example P(she PRON) = 0.1 P(PRON START) = 0.5 P(can AUX) = 0.2 P(AUX PRON) = 0.2 P(can NOUN) = P(NOUN PRON) = P(run VERB) = 0.01 P(VERB AUX) = 0.5 P(run NOUN) = P(NOUN AUX) = P(VERB NOUN) = 0.2 P(NOUN NOUN) = 0.1 Natural Language Processing 12(14)

29 Example P(she PRON) = 0.1 P(PRON START) = 0.5 P(can AUX) = 0.2 P(AUX PRON) = 0.2 P(can NOUN) = P(NOUN PRON) = P(run VERB) = 0.01 P(VERB AUX) = 0.5 P(run NOUN) = P(NOUN AUX) = P(VERB NOUN) = 0.2 P(NOUN NOUN) = 0.1 P(she/PRON can/aux run/verb) = = Natural Language Processing 12(14)

30 Example P(she PRON) = 0.1 P(PRON START) = 0.5 P(can AUX) = 0.2 P(AUX PRON) = 0.2 P(can NOUN) = P(NOUN PRON) = P(run VERB) = 0.01 P(VERB AUX) = 0.5 P(run NOUN) = P(NOUN AUX) = P(VERB NOUN) = 0.2 P(NOUN NOUN) = 0.1 P(she/PRON can/aux run/verb) = = P(she/PRON can/noun run/noun) = = Natural Language Processing 12(14)

31 Fundamental Problems I Decoding: I How do we compute the best tag sequence given parameters? I Learning: I How do we estimate the parameters? Natural Language Processing 13(14)

32 Quiz 2 I Consider this simple HMM for tagging: P(she PRON) = 0.1 P(PRON START) = 0.5 P(can AUX) = 0.2 P(AUX PRON) = 0.2 P(can NOUN) = P(NOUN PRON) = P(run VERB) = 0.01 P(VERB AUX) = 0.5 P(run NOUN) = P(NOUN AUX) = P(VERB NOUN) = 0.2 P(NOUN NOUN) = 0.1 I Which of the following statements are true? 1. The probability that can is a NOUN is The probability that the word after an AUX is not a VERB is P(she/PRON can/aux) > P(she/PRON can/noun) Natural Language Processing 14(14)