Part-of-Speech (POS) tagging See Eric Brill “Part-of-speech tagging”. Chapter 17 of R Dale, H Moisl & H Somers (eds) Handbook of Natural Language Processing, New York (2000): Marcel Dekker D Jurafsky & JH Martin: Speech and Language Processing, Upper Saddle River NJ (2000): Prentice Hall, Chapter 8 CD Manning & H Schütze: Foundations of Statistical Natural Language Processing, Cambridge, Mass (1999): MIT Press, Chapter 10. [skip the maths bits if too daunting]

2/24 Word categories A.k.a. parts of speech (POSs) Important and useful to identify words by their POS –To distinguish homonyms –To enable more general word searches POS familiar (?) from school and/or language learning (noun, verb, adjective, etc.)

3/24 Word categories Recall that we distinguished –open-class categories (noun, verb, adjective, adverb) –Closed-class categories (preposition, determiner, pronoun, conjunction, …) While the big four are fairly clearcut, it is less obvious exactly what and how many closed-class categories there may be

4/24 POS tagging Labelling words for POS can be done by –dictionary lookup –morphological analysis –“tagging” Identifying POS can be seen as a prerequisite to parsing, and/or a process in its own right However, there are some differences: –Parsers often work with the most simple set of word categories, subcategorized by feature (or attribute- value) schemes –Indeed the parsing procedure may contribute to the disambiguation of homonyms

5/24 POS tagging POS tagging, per se, aims to identify word-category information somewhat independently of sentence structure … … and typically uses rather different means POS tags are generally shown as labels on words: John/NPN saw/VB the/AT book/NCN on/PRP the/AT table/NN./PNC

6/24 What is a tagger? Lack of distinction between … –Software which allows you to create something you can then use to tag input text, e.g. “Brill’s tagger” –The result of running such software, e.g. a tagger for English (based on the such-and-such corpus) Taggers (even rule-based ones) are almost invariably trained on a given corpus “Tagging” usually understood to mean “POS tagging”, but you can have other types of tags (eg semantic tags)

7/24 Tagging vs. parsing Once tagger is “trained”, process consists straightforward look-up, plus local context (and sometimes morphology) Tagger will attempt to assign a tag to unknown words, and to disambiguate homographs “Tagset” (list of categories) usually larger with more distinctions than categories used in parsing

8/24 Tagset Parsing usually has basic word-categories, whereas tagging makes more subtle distinctions E.g. noun sg vs pl vs genitive, common vs proper, +is, +has, … and all combinations Parser uses maybe categories, tagger may use

9/24 Simple taggers Default tagger has one tag per word, and assigns it on the basis of dictionary lookup –Tags may indicate ambiguity but not resolve it, e.g. NVB for noun-or-verb Words may be assigned different tags with associated probabilities –Tagger will assign most probable tag unless –there is some way to identify when a less probable tag is in fact correct Tag sequences may be defined by regular expressions, and assigned probabilities (including 0 for illegal sequences – negative rules)

10/24 Rule-based taggers Earliest type of tagging: two stages 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 cf transformation-based tagging, below

11/24 How do they work? Tagger must be “trained” Many different techniques, but typically … Small “training corpus” hand-tagged Tagging rules learned automatically Rules define most likely sequence of tags Rules based on –Internal evidence (morphology) –External evidence (context) –Probabilities

12/24 What probabilities do we have to learn? (a) Individual word probabilities: Probability that a given tag t is appropriate for a given word w –Easy (in principle): learn from training corpus: –Problem of “sparse data”: Add a small amount to each calculation, so we get no zeros run occurs 4800 times in the training corpus: 3600 times as a verb, 1200 times as a noun: P(verb|run) = 0.75

13/24 (b) Tag sequence probability: Probability that a given tag sequence t 1,t 2,…,t n is appropriate for a given word sequence w 1,w 2,…,w n –P(t 1,t 2,…,t n | w 1,w 2,…,w n ) = ??? –Too hard to calculate for entire sequence: P(t 1,t 2,t 3,t 4,...) = P(t 2 |t 1 )  P(t 3 |t 1,t 2 )  P(t 4 |t 1,t 2,t 3 )  … –Subsequence is more tractable –Sequence of 2 or 3 should be enough: Bigram model: P(t 1,t 2 ) = P(t 2 |t 1 ) Trigram model: P(t 1,t 2,t 3 ) = P(t 2 |t 1 )  P(t 3 |t 2 ) N-gram model:

14/24 More complex taggers Bigram taggers assign tags on the basis of sequences of two words (usually assigning tag to word n on the basis of word n-1 ) An nth-order tagger assigns tags on the basis of sequences of n words As the value of n increases, so does the complexity of the statistical calculation involved in comparing probability combinations

15/24 Stochastic taggers Nowadays, pretty much all taggers are statistics-based and have been since 1980s (or even earlier... Some primitive algorithms were already published in 60s and 70s) Most common is based on Hidden Markov Models (also found in speech processing, etc.)

16/24 (Hidden) Markov Models Probability calculations imply Markov models: we assume that P(t|w) is dependent only on the (or, a sequence of) previous word(s) (Informally) Markov models are the class of probabilistic models that assume we can predict the future without taking too much account of the past Markov chains can be modelled by finite state automata: the next state in a Markov chain is always dependent on some finite history of previous states Model is “hidden” if it is actually a succession of Markov models, whose intermediate states are of no interest

17/24 Supervised vs unsupervised training Learning tagging rules from a marked-up corpus (supervised learning) gives very good results (98% accuracy) –Though assigning most probable tag, and “proper noun” to unknowns will give 90% But it depends on having a corpus already marked up to a high quality If this is not available, we have to try something else: –“forward-backward” algorithm –A kind of “bootstrapping” approach

18/24 Forward-backward (Baum-Welch) algorithm Start with initial probabilities –If nothing known, assume all Ps equal Adjust the individual probabilities so as to increase the overall probability. Re-estimate the probabilities on the basis of the last iteration Continue until convergence –i.e. there is no improvement, or improvement is below a threshold All this can be done automatically

19/24 Transformation-based tagging Eric Brill (1993) Start from an initial tagging, and apply a series of transformations Transformations are learned as well, from the training data Captures the tagging data in much fewer parameters than stochastic models The transformations learned (often) have linguistic “reality”

20/24 Transformation-based tagging Three stages: –Lexical look-up –Lexical rule application for unknown words –Contextual rule application to correct mis-tags Painting analogy

21/24 Transformation-based learning Change tag a to b when: –Internal evidence (morphology) –Contextual evidence One or more of the preceding/following words has a specific tag One or more of the preceding/following words is a specific word One or more of the preceding/following words has a certain form Order of rules is important –Rules can change a correct tag into an incorrect tag, so another rule might correct that “mistake”

22/24 Transformation-based tagging: examples if a word is currently tagged NN, and has a suffix of length 1 which consists of the letter 's', change its tag to NNS if a word has a suffix of length 2 consisting of the letter sequence 'ly', change its tag to RB (regardless of the initial tag) change VBN to VBD if previous word is tagged as NN Change VBD to VBN if previous word is ‘by’

23/24 Transformation-based tagging: example Booth/NP killed/VBN Abraham/NP Lincoln/NP Abraham/NP Lincoln/NP was/BEDZ shot/VBD by/BY Booth/NP He/PPS witnessed/VBD Lincoln/NP killed/VBN by/BY Booth/NP Example after lexical lookup Booth/NP killed/VBD Abraham/NP Lincoln/NP Abraham/NP Lincoln/NP was/BEDZ shot/VBN by/BY Booth/NP He/PPS witnessed/VBD Lincoln/NP killed/VBN by/BY Booth/NP Example after application of contextual rule ’vbd vbn NEXTWORD by’ Booth/NP killed/VBD Abraham/NP Lincoln/NP Abraham/NP Lincoln/NP was/BEDZ shot/VBD by/BY Booth/NP He/PPS witnessed/VBD Lincoln/NP killed/VBD by/BY Booth/NP Example after application of contextual rule ’vbn vbd PREVTAG np’

24/24 Tagging – final word Many taggers now available for download Sometimes not clear whether “tagger” means –Software enabling you to build a tagger given a corpus –An already built tagger for a given language Because a given tagger (2 nd sense) will have been trained on some corpus, it will be biased towards that (kind of) corpus –Question of goodness of match between original training corpus and material you want to use the tagger on