Lecture 10 NLTK POS Tagging Part 3 Topics Taggers Rule Based Taggers Probabilistic Taggers Transformation Based Taggers - Brill Supervised learning Readings: Chapter 5.4-? February 18, 2013 CSCE 771 Natural Language Processing

– 2 – CSCE 771 Spring 2011 Overview Last Time Overview of POS TagsToday Part of Speech Tagging Parts of Speech Rule Based taggers Stochastic taggers Transformational taggersReadings Chapter ?

– 3 – CSCE 771 Spring 2011 brown_lrnd_tagged = brown.tagged_words(categories='learned', simplify_tags=True) tags = [b[1] for (a, b) in nltk.ibigrams(brown_lrnd_tagged) if a[0] == 'often'] fd = nltk.FreqDist(tags) print fd.tabulate() VN V VD ADJ DET ADV P, CNJ. TO VBZ VG WH

– 4 – CSCE 771 Spring 2011 highly ambiguous words >>> brown_news_tagged = brown.tagged_words(categories='news', simplify_tags=True) >>> data = nltk.ConditionalFreqDist((word.lower(), tag)... for (word, tag) in brown_news_tagged) >>> data = nltk.ConditionalFreqDist((word.lower(), tag)... for (word, tag) in brown_news_tagged) >>> for word in data.conditions():... if len(data[word]) > 3:... tags = data[word].keys()... print word, ' '.join(tags)... best ADJ ADV NP V better ADJ ADV V DET ….

– 5 – CSCE 771 Spring 2011 Tag Package

– 6 – CSCE 771 Spring 2011 Python's Dictionary Methods:.

– 7 – CSCE 771 Spring Automatic Tagging Training set Test set ### setup import nltk, re, pprint from nltk.corpus import brown brown_tagged_sents = brown.tagged_sents(categories='news') brown_sents = brown.sents(categories='news')

– 8 – CSCE 771 Spring 2011 Default.tagger  NN tags = [tag for (word, tag) in brown.tagged_words(categories='news')] print nltk.FreqDist(tags).max() raw = 'I do not like green eggs and ham, I …Sam I am!' tokens = nltk.word_tokenize(raw) default_tagger = nltk.DefaultTagger('NN') print default_tagger.tag(tokens) [('I', 'NN'), ('do', 'NN'), ('not', 'NN'), ('like', 'NN'), … print default_tagger.evaluate(brown_tagged_sents)

– 9 – CSCE 771 Spring 2011 Tagger2: regexp_tagger patterns = [ (r'.*ing$', 'VBG'), # gerunds (r'.*ing$', 'VBG'), # gerunds (r'.*ed$', 'VBD'), # simple past (r'.*ed$', 'VBD'), # simple past (r'.*es$', 'VBZ'), # 3rd singular present (r'.*es$', 'VBZ'), # 3rd singular present (r'.*ould$', 'MD'), # modals (r'.*ould$', 'MD'), # modals (r'.*\'s$', 'NN$'), # possessive nouns (r'.*\'s$', 'NN$'), # possessive nouns (r'.*s$', 'NNS'), # plural nouns (r'.*s$', 'NNS'), # plural nouns (r'^-?[0-9]+(.[0-9]+)?$', 'CD'), # cardinal numbers (r'^-?[0-9]+(.[0-9]+)?$', 'CD'), # cardinal numbers (r'.*', 'NN') # nouns (default) (r'.*', 'NN') # nouns (default)] regexp_tagger = nltk.RegexpTagger(patterns)

– 10 – CSCE 771 Spring 2011 Evaluate regexp_tagger regexp_tagger = nltk.RegexpTagger(patterns) print regexp_tagger.tag(brown_sents[3]) [('``', 'NN'), ('Only', 'NN'), ('a', 'NN'), ('relative', 'NN'), … print regexp_tagger.evaluate(brown_tagged_sents)

– 11 – CSCE 771 Spring 2011 Unigram Tagger: 100 Most Freq tag fd = nltk.FreqDist(brown.words(categories='news')) cfd = nltk.ConditionalFreqDist(brown.tagged_words(categories='news')) most_freq_words = fd.keys()[:100] likely_tags = dict((word, cfd[word].max()) for word in most_freq_words) baseline_tagger = nltk.UnigramTagger(model=likely_tags) print baseline_tagger.evaluate(brown_tagged_sents)

– 12 – CSCE 771 Spring 2011 Likely_tags; Backoff to NN sent = brown.sents(categories='news')[3] baseline_tagger.tag(sent) ('Only', 'NN'), ('a', 'NN'), ('relative', 'NN'), ('handful', 'NN'), ('of', 'NN'), baseline_tagger = nltk.UnigramTagger(model=likely_tags, backoff=nltk.DefaultTagger('NN')) backoff=nltk.DefaultTagger('NN')) print baseline_tagger.tag(sent) 'Only', 'NN'), ('a', 'AT'), ('relative', 'NN'), ('handful', 'NN'), ('of', 'IN'), print baseline_tagger.evaluate(brown_tagged_sents)

– 13 – CSCE 771 Spring 2011 Performance of Easy Taggers. TaggerPerformanceComment NN tagger0.13 Regexp tagger Most Freq tag0.46 Likely_tags; Backoff to NN 0.58

– 14 – CSCE 771 Spring 2011 def performance(cfd, wordlist): lt = dict((word, cfd[word].max()) for word in wordlist) lt = dict((word, cfd[word].max()) for word in wordlist) baseline_tagger = nltk.UnigramTagger(model=lt, backoff=nltk.DefaultTagger('NN')) baseline_tagger = nltk.UnigramTagger(model=lt, backoff=nltk.DefaultTagger('NN')) return baseline_tagger.evaluate(brown.tagged_sents(categ ories='news')) return baseline_tagger.evaluate(brown.tagged_sents(categ ories='news'))

– 15 – CSCE 771 Spring 2011 Display def display(): import pylab words_by_freq = list(nltk.FreqDist(brown.words(categories='news'))) words_by_freq = list(nltk.FreqDist(brown.words(categories='news'))) cfd = nltk.ConditionalFreqDist(brown.tagged_words(categ ories='news')) cfd = nltk.ConditionalFreqDist(brown.tagged_words(categ ories='news')) sizes = 2 ** pylab.arange(15) sizes = 2 ** pylab.arange(15) perfs = [performance(cfd, words_by_freq[:size]) for size in sizes] perfs = [performance(cfd, words_by_freq[:size]) for size in sizes] pylab.plot(sizes, perfs, '-bo') pylab.plot(sizes, perfs, '-bo') pylab.title('Lookup Tagger Perf. vs Model Size') pylab.title('Lookup Tagger Perf. vs Model Size') pylab.xlabel('Model Size') pylab.xlabel('Model Size') pylab.ylabel('Performance') pylab.ylabel('Performance') pylab.show() pylab.show()

– 16 – CSCE 771 Spring 2011 Error !? Traceback (most recent call last): File "C:/Users/mmm/Documents/Courses/771/Python771/ ch05.4.py", line 70, in File "C:/Users/mmm/Documents/Courses/771/Python771/ ch05.4.py", line 70, in import pylab import pylab ImportError: No module named pylab google (download pylab)  scipy ??

– 17 – CSCE 771 Spring N-gram Tagging from nltk.corpus import brown brown_tagged_sents = brown.tagged_sents(categories='news') brown_sents = brown.sents(categories='news') unigram_tagger = nltk.UnigramTagger(brown_tagged_sents) print unigram_tagger.tag(brown_sents[2007]) [('Various', 'JJ'), ('of', 'IN'), ('the', 'AT'), ('apartments', 'NNS'), ('are', 'BER'), ('of', 'IN'), print unigram_tagger.evaluate(brown_tagged_sents)

– 18 – CSCE 771 Spring 2011 Dividing into Training/Test Sets size = int(len(brown_tagged_sents) * 0.9) print size 4160 train_sents = brown_tagged_sents[:size] test_sents = brown_tagged_sents[size:] unigram_tagger = nltk.UnigramTagger(train_sents) print unigram_tagger.evaluate(test_sents)

– 19 – CSCE 771 Spring 2011 bigram_tagger 1rst try -- bigram_tagger = nltk.BigramTagger(train_sents) print "bigram_tagger.tag-2007", bigram_tagger.tag(brown_sents[2007]) bigram_tagger.tag-2007 [('Various', 'JJ'), ('of', 'IN'), ('the', 'AT'), ('apartments', 'NNS'), ('are', 'BER') unseen_sent = brown_sents[4203] print "bigram_tagger.tag-4203", bigram_tagger.tag(unseen_sent) bigram_tagger.tag-4203 [('The', 'AT'), ('is', 'BEZ'), ('13.5', None), ('million', None), (',', None), ('divided', None), print bigram_tagger.evaluate(test_sents) not too good

– 20 – CSCE 771 Spring 2011 Backoff bigram  unigram  NN t0 = nltk.DefaultTagger('NN') t1 = nltk.UnigramTagger(train_sents, backoff=t0) t2 = nltk.BigramTagger(train_sents, backoff=t1) print t2.evaluate(test_sents)

– 21 – CSCE 771 Spring 2011 Your turn: tri  bi  uni  NN

– 22 – CSCE 771 Spring 2011 Tagging Unknown Words Our approach to tagging unknown words still uses backoff to a regular-expression tagger or a default tagger. These are unable to make use of context. Thus, if our tagger encountered the word blog, not seen during training, it would assign it the same tag, regardless of whether this word appeared in the context the blog or to blog. How can we do better with these unknown words, or out-of-vocabulary items? A useful method to tag unknown words based on context is to limit the vocabulary of a tagger to the most frequent n words, and to replace every other word with a special word UNK using the method shown in 5.3. During training, a unigram tagger will probably learn that UNK is usually a noun. However, the n-gram taggers will detect contexts in which it has some other tag. For example, if the preceding word is to (tagged TO), then UNK will probably be tagged as a verb. 5.3

– 23 – CSCE 771 Spring 2011 Serialization = pickle Saving Object serialization from cPickle import dump output=open('t2.pkl', 'wb') dump(t2, output, -1) output.close() Loading from cPickle import load input = open('t2.pkl', 'rb') tagger = load(input) input.close()

– 24 – CSCE 771 Spring 2011 Performance Limitations

– 25 – CSCE 771 Spring 2011 text = """The board's action shows what free enterprise is up against in our complex maze of regulatory laws.""" is up against in our complex maze of regulatory laws.""" tokens = text.split() tagger.tag(tokens) cfd = nltk.ConditionalFreqDist( ((x[1], y[1], z[0]), z[1]) ((x[1], y[1], z[0]), z[1]) for sent in brown_tagged_sents for sent in brown_tagged_sents for x, y, z in nltk.trigrams(sent)) for x, y, z in nltk.trigrams(sent)) ambiguous_contexts = [c for c in cfd.conditions() if len(cfd[c]) > 1] print sum(cfd[c].N() for c in ambiguous_contexts) / cfd.N()

– 26 – CSCE 771 Spring 2011 Confusion Matrix test_tags = [tag for sent in brown.sents(categories='editorial') for (word, tag) in t2.tag(sent)] for (word, tag) in t2.tag(sent)] gold_tags = [tag for (word, tag) in brown.tagged_words(categories='editorial')] print nltk.ConfusionMatrix(gold_tags, test_tags) overwhelming output

– 27 – CSCE 771 Spring 2011

– 28 – CSCE 771 Spring 2011 nltk.tag.brill.demo() Loading tagged data... Done loading. Training unigram tagger: [accuracy: ] [accuracy: ] Training bigram tagger: [accuracy: ] [accuracy: ] Training Brill tagger on 1600 sentences... Finding initial useful rules... Found 9757 useful rules. Found 9757 useful rules.

– 29 – CSCE 771 Spring 2011 S F r O | Score = Fixed - Broken S F r O | Score = Fixed - Broken c i o t | R Fixed = num tags changed incorrect -> correct c i o t | R Fixed = num tags changed incorrect -> correct o x k h | u Broken = num tags changed correct -> incorrect o x k h | u Broken = num tags changed correct -> incorrect r e e e | l Other = num tags changed incorrect -> incorrect r e e e | l Other = num tags changed incorrect -> incorrect e d n r | e e d n r | e | WDT -> IN if the tag of words i+1...i+2 is 'DT' | WDT -> IN if the tag of words i+1...i+2 is 'DT' | IN -> RB if the text of the following word is | IN -> RB if the text of the following word is | 'well' | 'well' | WDT -> IN if the tag of the preceding word is | WDT -> IN if the tag of the preceding word is | 'NN', and the tag of the following word is 'NNP' | 'NN', and the tag of the following word is 'NNP' | RBR -> JJR if the tag of words i+1...i+2 is 'NNS' | RBR -> JJR if the tag of words i+1...i+2 is 'NNS' | WDT -> IN if the tag of words i+1...i+2 is 'NNS' | WDT -> IN if the tag of words i+1...i+2 is 'NNS'

– 30 – CSCE 771 Spring | WDT -> IN if the tag of the preceding word is | WDT -> IN if the tag of the preceding word is | 'NN', and the tag of the following word is 'PRP' | 'NN', and the tag of the following word is 'PRP' | WDT -> IN if the tag of words i+1...i+3 is 'VBG' | WDT -> IN if the tag of words i+1...i+3 is 'VBG' | RB -> IN if the tag of the preceding word is 'NN', | RB -> IN if the tag of the preceding word is 'NN', | and the tag of the following word is 'DT' | and the tag of the following word is 'DT' | RBR -> JJR if the tag of the following word is | RBR -> JJR if the tag of the following word is | 'NN' | 'NN' | VBP -> VB if the tag of words i-3...i-1 is 'MD' | VBP -> VB if the tag of words i-3...i-1 is 'MD' | NNS -> NN if the text of the preceding word is | NNS -> NN if the text of the preceding word is | 'one' | 'one' | RP -> RB if the text of words i-3...i-1 is 'were' | RP -> RB if the text of words i-3...i-1 is 'were' | VBP -> VB if the text of words i-2...i-1 is "n't" | VBP -> VB if the text of words i-2...i-1 is "n't" Brill accuracy: Done; rules and errors saved to rules.yaml and errors.out.

– 31 – CSCE 771 Spring 2011