CSA2050: Introduction to Computational Linguistics Part of Speech (POS) Tagging I Introduction Tagsets Approaches

April 2005CLINT Lecture IV2 Acknowledgment Most slides taken from Bonnie Dorr’s course notes: In turn based on Jurafsky & Martin Chapter 8

Bibliography R. Weischedel, R. Schwartz, J. Palmucci, M. Meteer, L. Ramshaw, Coping with Ambiguity and Unknown Words through Probabilistic Models, Computational Linguistics 19.2, pp ,1993 [pdf]pdf Samuelsson, C., Morphological tagging based entirely on Bayesian inference, in 9th Nordic Conference on Computational Linguistics, NODALIDA-93, Stockholm, (see [html])html A. Ratnaparkhi, A maximum entropy model for part of speech tagging. Proceedings of the Conference on Empirical Methods in Natural Language, 1996 Processing [pdf]pdf

April 2005CLINT Lecture IV4 Outline The tagging task Tagsets Three different approaches

April 2005CLINT Lecture IV5 Definition: PoS-Tagging “ Part-of-Speech Tagging is the process of assigning a part-of-speech or other lexical class marker to each word in a corpus ” (Jurafsky and Martin) the girl kissed the boy on the cheek WORDS TAGS N V P DET

April 2005CLINT Lecture IV6 Motivation Corpus analysis of tagged corpora yields useful information Speech synthesis — pronunciation CONtent (N) vs. conTENT (Adj) Speech recognition — word class-based N- grams predict category of next word. Information retrieval stemming selection of high-content words Word-sense disambiguation

English Parts of Speech 1. Pronoun: any substitute for a noun or noun phrase 2. Adjective: any qualifier of a noun 3. Verb: any action or state of being 4. Adverb: any qualifier of an adjective verb 5. Preposition: any establisher of relation and syntactic context 6. Conjunction: any syntactic connector 7. Interjection: any emotional greeting (or "exclamation"),

April 2005CLINT Lecture IV8 Tagsets: how detailed? Swedish SUC25 Penn Treebank46 German STTS50 Lancaster BNC61 Lancaster Full146

April 2005CLINT Lecture IV9 Penn Treebank Tagset PRP PRP$

April 2005CLINT Lecture IV10 Example of Penn Treebank Tagging of Brown Corpus Sentence The/DT grand/JJ jury/NN commented/VBD on/IN a/DT number/NN of/IN other/JJ topics/NNS./. VB DT NN. Book that flight. VBZ DT NN VB NN ? Does that flight serve dinner ?

2 Problems Multiple tags for the same word Unknown words

April 2005CLINT Lecture IV12 Multiple tags for the same word 1. He can can a can. 2. I can light a fire and you can open a can of beans. Now the can is open, and we can eat in the light of the fire. 3. Flying planes can be dangerous.

April 2005CLINT Lecture IV13 Multiple tags for the same word Words often belong to more than one word class: this This is a nice day = PRP (pronoun) This day is nice = DT (determiner) You can go this far = RB (adverb) Many of the most common words (by volume of text) are ambiguous

April 2005CLINT Lecture IV14 How Hard is the Tagging Task? In the Brown Corpus 11.5% of word types are ambiguous 40% of word tokens are ambiguous Most words in English are unambiguous. Many of the most common words are ambiguous. Typically ambiguous tags are not equally probable.

April 2005CLINT Lecture IV15 Word Class Ambiguity (in the Brown Corpus) Unambiguous (1 tag): 35,340 types Ambiguous (2-7 tags): 4,100 types. 2 tags3,760 3 tags264 4 tags61 5 tags12 6 tags2 7 tags1 (Derose, 1988)

April 2005CLINT Lecture IV16 3 Approaches to Tagging 1.Rule-Based Tagger: ENCG Tagger (Voutilainen 1995,1999) 2.Stochastic Tagger: HMM-based Tagger 3.Transformation-Based Tagger: Brill Tagger (Brill 1995)

Unknown Words 1. Assume all unknown word is ambiguous amongst all possible tags Advantage: simplicity Disadvantage: ignores the fact that unknown words are unlikely to be closed class 2. Assume that probability distribution of unknown words is same as words that have been seen just once. 3. Make use of morphological information

Combining Features The last method makes use of different features, e.g. ending in -ed (suggest verb) or initial capital (suggests proper noun). Typically, a given tag is correlated with a combination of such features. These have to be incorporated into the statistical model.

Combining Tag-Predicting Features in Unknown Words HMM Models Weischedel et. al. (1993): for each feature f and tag t (e.g. proper noun) build a probability estimator p(f|t). Assume independence and multiply probabilities together Samuelsson (1993), rather than preselecting features, considers all possible suffixes up to length 10 as features for predicting tags

Combining Tag-Predicting Features in Unknown Words Maximum Entropy (ME) Models. A ME model is a classifier which assigns a class to an observation by computing a probability from an exponential function of a weighted set of features of the observation An MEMM uses the Viterbi Algorithm to extend the application of ME to labelling a sequence of observations. For further details see Ratnaparkhi (1996)

Summary External parameters to the tagging task are (i) the size of the chosen tagset and (ii) the coverage of the lexicon which gives possible tags to words. Two main problems: (i) disambiguation of tags and (ii) dealing with unknown words Several methods are available for dealing with (ii): HMMs and MEMMs