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284 lines
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284 lines
11 KiB
Plaintext
.. Copyright (C) 2001-2020 NLTK Project
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.. For license information, see LICENSE.TXT
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==============
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Collocations
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==============
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Overview
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~~~~~~~~
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Collocations are expressions of multiple words which commonly co-occur. For
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example, the top ten bigram collocations in Genesis are listed below, as
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measured using Pointwise Mutual Information.
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>>> import nltk
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>>> from nltk.collocations import *
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>>> bigram_measures = nltk.collocations.BigramAssocMeasures()
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>>> trigram_measures = nltk.collocations.TrigramAssocMeasures()
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>>> fourgram_measures = nltk.collocations.QuadgramAssocMeasures()
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>>> finder = BigramCollocationFinder.from_words(
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... nltk.corpus.genesis.words('english-web.txt'))
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>>> finder.nbest(bigram_measures.pmi, 10) # doctest: +NORMALIZE_WHITESPACE
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[('Allon', 'Bacuth'), ('Ashteroth', 'Karnaim'), ('Ben', 'Ammi'),
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('En', 'Mishpat'), ('Jegar', 'Sahadutha'), ('Salt', 'Sea'),
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('Whoever', 'sheds'), ('appoint', 'overseers'), ('aromatic', 'resin'),
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('cutting', 'instrument')]
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While these words are highly collocated, the expressions are also very
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infrequent. Therefore it is useful to apply filters, such as ignoring all
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bigrams which occur less than three times in the corpus:
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>>> finder.apply_freq_filter(3)
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>>> finder.nbest(bigram_measures.pmi, 10) # doctest: +NORMALIZE_WHITESPACE
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[('Beer', 'Lahai'), ('Lahai', 'Roi'), ('gray', 'hairs'),
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('Most', 'High'), ('ewe', 'lambs'), ('many', 'colors'),
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('burnt', 'offering'), ('Paddan', 'Aram'), ('east', 'wind'),
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('living', 'creature')]
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We may similarly find collocations among tagged words:
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>>> finder = BigramCollocationFinder.from_words(
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... nltk.corpus.brown.tagged_words('ca01', tagset='universal'))
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>>> finder.nbest(bigram_measures.pmi, 5) # doctest: +NORMALIZE_WHITESPACE
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[(('1,119', 'NUM'), ('votes', 'NOUN')),
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(('1962', 'NUM'), ("governor's", 'NOUN')),
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(('637', 'NUM'), ('E.', 'NOUN')),
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(('Alpharetta', 'NOUN'), ('prison', 'NOUN')),
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(('Bar', 'NOUN'), ('Association', 'NOUN'))]
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Or tags alone:
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>>> finder = BigramCollocationFinder.from_words(t for w, t in
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... nltk.corpus.brown.tagged_words('ca01', tagset='universal'))
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>>> finder.nbest(bigram_measures.pmi, 10) # doctest: +NORMALIZE_WHITESPACE
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[('PRT', 'VERB'), ('PRON', 'VERB'), ('ADP', 'DET'), ('.', 'PRON'), ('DET', 'ADJ'),
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('CONJ', 'PRON'), ('ADP', 'NUM'), ('NUM', '.'), ('ADV', 'ADV'), ('VERB', 'ADV')]
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Or spanning intervening words:
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>>> finder = BigramCollocationFinder.from_words(
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... nltk.corpus.genesis.words('english-web.txt'),
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... window_size = 20)
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>>> finder.apply_freq_filter(2)
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>>> ignored_words = nltk.corpus.stopwords.words('english')
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>>> finder.apply_word_filter(lambda w: len(w) < 3 or w.lower() in ignored_words)
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>>> finder.nbest(bigram_measures.likelihood_ratio, 10) # doctest: +NORMALIZE_WHITESPACE
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[('chief', 'chief'), ('became', 'father'), ('years', 'became'),
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('hundred', 'years'), ('lived', 'became'), ('king', 'king'),
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('lived', 'years'), ('became', 'became'), ('chief', 'chiefs'),
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('hundred', 'became')]
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Finders
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~~~~~~~
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The collocations package provides collocation finders which by default
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consider all ngrams in a text as candidate collocations:
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>>> text = "I do not like green eggs and ham, I do not like them Sam I am!"
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>>> tokens = nltk.wordpunct_tokenize(text)
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>>> finder = BigramCollocationFinder.from_words(tokens)
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>>> scored = finder.score_ngrams(bigram_measures.raw_freq)
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>>> sorted(bigram for bigram, score in scored) # doctest: +NORMALIZE_WHITESPACE
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[(',', 'I'), ('I', 'am'), ('I', 'do'), ('Sam', 'I'), ('am', '!'),
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('and', 'ham'), ('do', 'not'), ('eggs', 'and'), ('green', 'eggs'),
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('ham', ','), ('like', 'green'), ('like', 'them'), ('not', 'like'),
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('them', 'Sam')]
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We could otherwise construct the collocation finder from manually-derived
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FreqDists:
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>>> word_fd = nltk.FreqDist(tokens)
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>>> bigram_fd = nltk.FreqDist(nltk.bigrams(tokens))
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>>> finder = BigramCollocationFinder(word_fd, bigram_fd)
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>>> scored == finder.score_ngrams(bigram_measures.raw_freq)
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True
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A similar interface is provided for trigrams:
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>>> finder = TrigramCollocationFinder.from_words(tokens)
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>>> scored = finder.score_ngrams(trigram_measures.raw_freq)
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>>> set(trigram for trigram, score in scored) == set(nltk.trigrams(tokens))
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True
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We may want to select only the top n results:
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>>> sorted(finder.nbest(trigram_measures.raw_freq, 2))
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[('I', 'do', 'not'), ('do', 'not', 'like')]
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Alternatively, we can select those above a minimum score value:
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>>> sorted(finder.above_score(trigram_measures.raw_freq,
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... 1.0 / len(tuple(nltk.trigrams(tokens)))))
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[('I', 'do', 'not'), ('do', 'not', 'like')]
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Now spanning intervening words:
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>>> finder = TrigramCollocationFinder.from_words(tokens)
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>>> finder = TrigramCollocationFinder.from_words(tokens, window_size=4)
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>>> sorted(finder.nbest(trigram_measures.raw_freq, 4))
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[('I', 'do', 'like'), ('I', 'do', 'not'), ('I', 'not', 'like'), ('do', 'not', 'like')]
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A closer look at the finder's ngram frequencies:
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>>> sorted(finder.ngram_fd.items(), key=lambda t: (-t[1], t[0]))[:10] # doctest: +NORMALIZE_WHITESPACE
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[(('I', 'do', 'like'), 2), (('I', 'do', 'not'), 2), (('I', 'not', 'like'), 2),
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(('do', 'not', 'like'), 2), ((',', 'I', 'do'), 1), ((',', 'I', 'not'), 1),
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((',', 'do', 'not'), 1), (('I', 'am', '!'), 1), (('Sam', 'I', '!'), 1),
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(('Sam', 'I', 'am'), 1)]
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A similar interface is provided for fourgrams:
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>>> finder_4grams = QuadgramCollocationFinder.from_words(tokens)
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>>> scored_4grams = finder_4grams.score_ngrams(fourgram_measures.raw_freq)
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>>> set(fourgram for fourgram, score in scored_4grams) == set(nltk.ngrams(tokens, n=4))
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True
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Filtering candidates
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~~~~~~~~~~~~~~~~~~~~
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All the ngrams in a text are often too many to be useful when finding
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collocations. It is generally useful to remove some words or punctuation,
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and to require a minimum frequency for candidate collocations.
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Given our sample text above, if we remove all trigrams containing personal
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pronouns from candidature, score_ngrams should return 6 less results, and
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'do not like' will be the only candidate which occurs more than once:
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>>> finder = TrigramCollocationFinder.from_words(tokens)
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>>> len(finder.score_ngrams(trigram_measures.raw_freq))
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14
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>>> finder.apply_word_filter(lambda w: w in ('I', 'me'))
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>>> len(finder.score_ngrams(trigram_measures.raw_freq))
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8
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>>> sorted(finder.above_score(trigram_measures.raw_freq,
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... 1.0 / len(tuple(nltk.trigrams(tokens)))))
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[('do', 'not', 'like')]
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Sometimes a filter is a function on the whole ngram, rather than each word,
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such as if we may permit 'and' to appear in the middle of a trigram, but
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not on either edge:
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>>> finder.apply_ngram_filter(lambda w1, w2, w3: 'and' in (w1, w3))
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>>> len(finder.score_ngrams(trigram_measures.raw_freq))
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6
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Finally, it is often important to remove low frequency candidates, as we
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lack sufficient evidence about their significance as collocations:
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>>> finder.apply_freq_filter(2)
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>>> len(finder.score_ngrams(trigram_measures.raw_freq))
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1
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Association measures
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~~~~~~~~~~~~~~~~~~~~
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A number of measures are available to score collocations or other associations.
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The arguments to measure functions are marginals of a contingency table, in the
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bigram case (n_ii, (n_ix, n_xi), n_xx)::
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w1 ~w1
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------ ------
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w2 | n_ii | n_oi | = n_xi
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------ ------
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~w2 | n_io | n_oo |
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------ ------
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= n_ix TOTAL = n_xx
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We test their calculation using some known values presented in Manning and
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Schutze's text and other papers.
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Student's t: examples from Manning and Schutze 5.3.2
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>>> print('%0.4f' % bigram_measures.student_t(8, (15828, 4675), 14307668))
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0.9999
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>>> print('%0.4f' % bigram_measures.student_t(20, (42, 20), 14307668))
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4.4721
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Chi-square: examples from Manning and Schutze 5.3.3
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>>> print('%0.2f' % bigram_measures.chi_sq(8, (15828, 4675), 14307668))
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1.55
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>>> print('%0.0f' % bigram_measures.chi_sq(59, (67, 65), 571007))
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456400
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Likelihood ratios: examples from Dunning, CL, 1993
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>>> print('%0.2f' % bigram_measures.likelihood_ratio(110, (2552, 221), 31777))
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270.72
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>>> print('%0.2f' % bigram_measures.likelihood_ratio(8, (13, 32), 31777))
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95.29
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Pointwise Mutual Information: examples from Manning and Schutze 5.4
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>>> print('%0.2f' % bigram_measures.pmi(20, (42, 20), 14307668))
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18.38
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>>> print('%0.2f' % bigram_measures.pmi(20, (15019, 15629), 14307668))
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0.29
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TODO: Find authoritative results for trigrams.
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Using contingency table values
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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While frequency counts make marginals readily available for collocation
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finding, it is common to find published contingency table values. The
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collocations package therefore provides a wrapper, ContingencyMeasures, which
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wraps an association measures class, providing association measures which
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take contingency values as arguments, (n_ii, n_io, n_oi, n_oo) in the
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bigram case.
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>>> from nltk.metrics import ContingencyMeasures
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>>> cont_bigram_measures = ContingencyMeasures(bigram_measures)
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>>> print('%0.2f' % cont_bigram_measures.likelihood_ratio(8, 5, 24, 31740))
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95.29
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>>> print('%0.2f' % cont_bigram_measures.chi_sq(8, 15820, 4667, 14287173))
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1.55
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Ranking and correlation
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~~~~~~~~~~~~~~~~~~~~~~~
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It is useful to consider the results of finding collocations as a ranking, and
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the rankings output using different association measures can be compared using
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the Spearman correlation coefficient.
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Ranks can be assigned to a sorted list of results trivially by assigning
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strictly increasing ranks to each result:
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>>> from nltk.metrics.spearman import *
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>>> results_list = ['item1', 'item2', 'item3', 'item4', 'item5']
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>>> print(list(ranks_from_sequence(results_list)))
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[('item1', 0), ('item2', 1), ('item3', 2), ('item4', 3), ('item5', 4)]
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If scores are available for each result, we may allow sufficiently similar
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results (differing by no more than rank_gap) to be assigned the same rank:
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>>> results_scored = [('item1', 50.0), ('item2', 40.0), ('item3', 38.0),
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... ('item4', 35.0), ('item5', 14.0)]
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>>> print(list(ranks_from_scores(results_scored, rank_gap=5)))
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[('item1', 0), ('item2', 1), ('item3', 1), ('item4', 1), ('item5', 4)]
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The Spearman correlation coefficient gives a number from -1.0 to 1.0 comparing
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two rankings. A coefficient of 1.0 indicates identical rankings; -1.0 indicates
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exact opposite rankings.
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>>> print('%0.1f' % spearman_correlation(
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... ranks_from_sequence(results_list),
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... ranks_from_sequence(results_list)))
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1.0
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>>> print('%0.1f' % spearman_correlation(
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... ranks_from_sequence(reversed(results_list)),
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... ranks_from_sequence(results_list)))
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-1.0
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>>> results_list2 = ['item2', 'item3', 'item1', 'item5', 'item4']
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>>> print('%0.1f' % spearman_correlation(
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... ranks_from_sequence(results_list),
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... ranks_from_sequence(results_list2)))
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0.6
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>>> print('%0.1f' % spearman_correlation(
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... ranks_from_sequence(reversed(results_list)),
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... ranks_from_sequence(results_list2)))
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-0.6
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