Categorizing and Tagging Words
Chapter 5 of the NLTK book
Plan for tonight
• Quiz
• Part of speech tagging
– Use of the Python dictionary data type
– Application of regular expressions
• Planning for the rest of the semester
Understanding text
• “Understanding” written and spoken text is
a very complicated process
– Distinguishing characteristic of humans
– Machines do not understand
• Trying to make machines behave as though they
understand has led to interesting insights into the
nature of understanding in humans
• Humans acquire an ability to characterize aspects of
language as used in an instance, which aids in
seeing meaning in the text or speech.
• If we want machines to process autonomously, we
have to provide explicitly the extra information that
humans learn to append to what is written or
spoken.
Categorizing and Tagging Words
• Chapter goals: address these questions:
– What are lexical categories and how are they
used in natural language processing?
– What is a good Python data structure for
storing words and their categories?
– How can we automatically tag each word of a
text with its word class?
• This necessarily introduces some of the
fundamental concepts of natural language
processing.
– Potential use in many application areas
Word classes
• aka Lexical categories.
• The particular collection of tags used is the
tagset
– How nice it would be if there were only one
tagset, general enough for all use!
• Tags for parts of speech identify nouns,
verbs, adverbs, adjectives, articles, etc
– Verbs are further tagged with tense, passive or
active voice, etc.
– There are lots of possibilities for the kinds of
tagging desired.
First tagging example
>>> text = nltk.word_tokenize("And now for something completely different")
>>> nltk.pos_tag(text)
[('And', 'CC'), ('now', 'RB'), ('for', 'IN'), ('something', 'NN'),
('completely', 'RB'), ('different', 'JJ')]
• First, tokenize
• The tags are cryptic. In this case,
– CC = coordinating conjunction
– RB = adverb
– IN = preposition
– NN = noun
– JJ = adjective
Example
import nltk
# from nltk.book import *
raw = raw_input("Enter a sentence: ")
text = nltk.word_tokenize(raw)
print text
result =
nltk.pos_tag(text)
print result
Enter a sentence: And now for something
completely different
['And', 'now', 'for', 'something', 'completely',
'different']
[('And', 'CC'), ('now', 'RB'), ('for', 'IN'),
('something', 'NN'), ('completely', 'RB'),
('different', 'JJ')]
Homonyms
• Different words that are spelled the
same, but have different meanings
– They may be pronounced differently, or not
– Tags cannot be assigned to the word
independently. Word in context required.
More difficult example
import nltk
# from nltk.book import *
raw = raw_input("Enter a sentence: ")
text = nltk.word_tokenize(raw)
print text
result =
nltk.pos_tag(text)
print result
Enter a sentence: They refuse to permit us to
obtain the refuse permit.
['They', 'refuse', 'to', 'permit', 'us', 'to',
'obtain', 'the', 'refuse', 'permit', '.']
[('They', 'PRP'), ('refuse', 'VBP'), ('to',
'TO'), ('permit', 'VB'), ('us', 'PRP'), ('to',
'TO'), ('obtain', 'VB'), ('the', 'DT'),
('refuse', 'NN'), ('permit', 'NN'), ('.', '.')]
What use is tagging?
• The goal is autonomous processing that
correctly communicates in
understandable language.
– Results:
•
•
•
•
automated telephone trees
Spoken directions in gps
Directions provided by mapping programs
Any system that uses free text input and
provides appropriate information responses
– shopping assistants, perhaps
– Medical diagnosis systems
– Many more
Spot check
• Describe a context in which program
“understanding” of free text is needed
and/or in which text or spoken response
– that was not preprogrammed – is
useful
Tagged Corpora
• In nltk, tagged token is a tuple
– (token, tag)
– This allows us to isolate the two components
and use each easily
• Tags in some corpora are done differently,
conversion available
>>> tagged_token = nltk.tag.str2tuple('fly/NN')
>>> tagged_token
('fly', 'NN')
>>> tagged_token[0]
'fly'
>>> tagged_token[1]
'NN'
Steps to token, tag tuples
• Original text has each word followed by / and
the tag.
– Change to tokens.
• Each word and / and tag is a token
– Separate each token into a word, tag tuple
>>> sent = '''
... The/AT grand/JJ jury/NN commented/VBD on/IN a/AT number/NN of/IN
... other/AP topics/NNS ,/, among/IN them/PPO the/AT Atlanta/NP and/CC
... Fulton/NP-tl County/NN-tl purchasing/VBG departments/NNS which/WDT it/PPS
... said/VBD ``/`` zr\\are/BER well/QL operated/VBN and/CC follow/VB generally/RB
... accepted/VBN practices/NNS which/WDT inure/VB to/IN the/AT best/JJT
... interest/NN of/IN both/ABX governments/NNS ''/'' ./.
... '''
>>> [nltk.tag.str2tuple(t) for t in sent.split()]
[('The', 'AT'), ('grand', 'JJ'), ('jury', 'NN'), ('commented', 'VBD'),
('on', 'IN'), ('a', 'AT'), ('number', 'NN'), ... ('.', '.')
Handling differing tagging styles
• Different corpora have different
conventions for tagging.
• NLTK harmonizes those and presents
them all as tuples
– tagged_words() method available for all
tagged text in the corpus.
• Note this will probably not work for arbitrarily
selected files. This is done for the files in the
corpus.
• NLTK simplified tagset
Simplified Tagset of NLTK
Other languages
• NLTK is used for languages other than
English, and for alphabets and writing
forms other than the Western
characters.
• See the example showing Four Indian
Languages (and tell me if they look
meaningful!)
Looking at text for part of speech
>>> from nltk.corpus import brown
>>> brown_news_tagged = brown.tagged_words(categories='news',
simplify_tags=True)
>>> tag_fd = nltk.FreqDist(tag for (word, tag) in brown_news_tagged)
>>> tag_fd.keys()
['N', 'P', 'DET', 'NP', 'V', 'ADJ', ',', '.', 'CNJ', 'PRO', 'ADV', 'VD', ...]
Patterns for nouns
• noun tags – N for common nouns, NP for
proper nouns
• What parts of speech occur before a noun?
– Construct list of bigrams
• word-tag pairs
– Frequency Distribution
>>> word_tag_pairs = nltk.bigrams(brown_news_tagged)
>>> list(nltk.FreqDist(a[1] for (a, b) in word_tag_pairs if b[1] == 'N'))
['DET', 'ADJ', 'N', 'P', 'NP', 'NUM', 'V', 'PRO', 'CNJ', '.', ',', 'VG', 'VN', ...]
A closer look
Let’s parse that carefully. What is in word_tag_pairs?
>>> word_tag_pairs = nltk.bigrams(brown_news_tagged)
>>> list(nltk.FreqDist(a[1] for (a, b) in word_tag_pairs if b[1] == 'N'))
['DET', 'ADJ', 'N', 'P', 'NP', 'NUM', 'V', 'PRO', 'CNJ', '.', ',', 'VG', 'VN', ...]
>>> word_tag_pairs[10]
One bigram (Atlanta’s recent) showing
(("Atlanta's", 'NP'), ('recent', 'ADJ')) each word with its tag
A slice of the word_tag_pairs with red
>>> word_tag_pairs[10:15] parentheses bracketing each tuple)
[(("Atlanta's", 'NP'), ('recent', 'ADJ')), (('recent', 'ADJ'), ('primary', 'N')), (('primary', 'N'),
('election', 'N')), (('election', 'N'), ('produced', 'VD')), (('produced', 'VD'), ('``', '``'))]
list(nltk.FreqDist(a[1] for (a, b) in word_tag_pairs if b[1] == 'N'))
>>> word_tag_pairs[10][1]
>>> word_tag_pairs[10][1][0]
('recent', 'ADJ')
'recent’
Patterns for verbs
• Looking for verbs in the news text and
sorting by frequency:
>>> wsj = nltk.corpus.treebank.tagged_words(simplify_tags=True)
>>> word_tag_fd = nltk.FreqDist(wsj)
>>> [word + "/" + tag for (word, tag) in word_tag_fd if tag.startswith('V')]
['is/V', 'said/VD', 'was/VD', 'are/V', 'be/V', 'has/V', 'have/V', 'says/V',
'were/VD', 'had/VD', 'been/VN', "'s/V", 'do/V', 'say/V', 'make/V', 'did/VD',
'rose/VD', 'does/V', 'expected/VN', 'buy/V', 'take/V', 'get/V', 'sell/V',
'help/V', 'added/VD', 'including/VG', 'according/VG', 'made/VN', 'pay/V', ...]
for <…> in <frequency distribution> if <condition> -- format for
conditional distribution
import nltk
# from nltk.book import *
wsj=nltk.corpus.treebank.tagged_words(simplify_tags=True)
word_tag_fd = nltk.FreqDist(wsj)
print "Frequency distribution:"
print word_tag_fd
verbs= [word+"/"+tag for (word,tag) in word_tag_fd if
tag.startswith('V')]
print "Verbs: "
print verbs[:25]
Frequency distribution:
<FreqDist: (',', ','): 4885, ('the', 'DET'): 4038, ('.', '.'): 3828, ('of', 'P'): 2319, ('to',
'TO'): 2161, ('a', 'DET'): 1874, ('in', 'P'): 1554, ('and', 'CNJ'): 1505, ('*-1', ''): 1123,
('0', ''): 1099, ...>
Verbs:
['is/V', 'said/VD', 'are/V', 'was/VD', 'be/V', 'has/V', 'have/V', 'says/V', 'were/VD',
'had/VD', 'been/VN', "'s/V", 'do/V', 'say/V', 'make/V', 'did/VD', 'rose/VD',
'does/V', 'expected/VN', 'buy/V', 'take/V', 'get/V', 'sell/V', 'help/V', 'added/VD']
Conditional distribution
• Recall that conditional distribution requires an
event and a condition.
– We can treat the word as the condition and the tag as
For yield and cut, show the parts of speech.
the event.
>>> cfd1 = nltk.ConditionalFreqDist(wsj)
>>> cfd1['yield'].keys()
import nltk
wsj=nltk.corpus.treebank.tagged_words(simplify_tags=True)
['V', 'N']
cfd1=nltk.ConditionalFreqDist(wsj)
>>> cfd1['cut'].keys()
word = raw_input("Enter the word to explore: ")
['V', 'VD', 'N', 'VN']
if word in cfd1:
wordkeys = cfd1[word].keys()
print wordkeys
else:
print "Word not found"
Distributions
• or reverse, so the words are the events and
we see the tags commonly associated with
given words:
>>> cfd2 = nltk.ConditionalFreqDist((tag, word) for\
(word, tag) in wsj)
>>> cfd2['VN'].keys()
['been', 'expected', 'made', 'compared', 'based',
'priced', 'used', 'sold',
'named', 'designed', 'held', 'fined', 'taken',
'paid', 'traded', 'said', ...]
Verb tense
• Clarifying past tense and past participle,
look at some words that are the same for
both and the words that are surrounding
them:
>>> [w for w in cfd1.conditions() if 'VD' in cfd1[w] and 'VN' in cfd1[w]]
['Asked', 'accelerated', 'accepted', 'accused', 'acquired', 'added', 'adopted', ...]
>>> idx1 = wsj.index(('kicked', 'VD'))
>>> wsj[idx1-4:idx1+1]
[('While', 'P'), ('program', 'N'), ('trades', 'N'), ('swiftly', 'ADV'),
('kicked', 'VD')]
>>> idx2 = wsj.index(('kicked', 'VN'))
>>> wsj[idx2-4:idx2+1]
[('head', 'N'), ('of', 'P'), ('state', 'N'), ('has', 'V'), ('kicked', 'VN')]
Spot check
• Get the list of past participles found with
cfd2[‘VN’].keys()
• Collect all the context for each by
showing word-tag pair immediately
before and immediately after each.
>>> cfd2 = nltk.ConditionalFreqDist((tag, word) for\
(word, tag) in wsj)
Adjectives and Adverbs
• … and other parts of speech. We can
find them all, examine their context, etc.
Example, looking at a word
• “often” how is it used in common
English usage?
>>> brown_learned_text = brown.words(categories='learned')
>>> sorted(set(b for (a, b) in nltk.ibigrams(brown_learned_text) if a == 'often'))
[',', '.', 'accomplished', 'analytically', 'appear', 'apt', 'associated', 'assuming',
'became', 'become', 'been', 'began', 'call', 'called', 'carefully', 'chose', ...]
This gives us a set of sorted words, from the bigrams in the indicated
text, where the first word is “often”
>>> 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)
>>> fd.tabulate()
VN
15
V
12
VD
8
DET
5
ADJ
5
ADV
4
P
4
CNJ
3
,
3
TO
1
VG
1
WH
1
VBZ
1
.
1
This gives us the distribution of the part of speech of the word following “often”
in sorted order
Finding patterns
• We saw how to use regular expressions to extract
patterns on words, now let’s extract patterns of
parts of speech.
– We look at each three-word phrase in the text, and
find <verb> to <verb>:
from nltk.corpus import brown
def process(sentence):
for (w1,t1), (w2,t2), (w3,t3) in \
nltk.trigrams(sentence):
if (t1.startswith('V') and t2 == 'TO' and \
t3.startswith('V')):
print w1, w2, w3
>>> for tagged_sent in brown.tagged_sents():
...
process(tagged_sent)
...
combined to achieve
continue to place
serve to protect
wanted to wait
allowed to place
expected to become
...
POS ambiguities
• Looking at how the words are tagged,
may help understand the tagging
>>> brown_news_tagged =
brown.tagged_words(categories='news',\
simplify_tags=True)
>>> 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)
...
Recall that a Conditional Frequency
best ADJ ADV NP V
Distribution
better ADJ ADV V DET
(ConditionalFreqDist()
close ADV ADJ V N
has an event and a condition.
cut V N VN VD
Each element of data has
even ADV DET ADJ V
data.conditions() and each condition has keys
data[…].keys()
Python Dictionary
• Allows mapping between arbitrary types
(not necessary to have a numeric index)
• Note the addition of defaultdict
– returns a value for a non-existing entry
• uses the default value for the type
• 0 for number, [] for empty list, etc.
– We can specify a default value to use
Tagging rare words
with default value
A lambda function is an unnamed function that can be defined and
used wherever a function object is required.
>>> alice = nltk.corpus.gutenberg.words('carrollalice.txt')
>>> vocab = nltk.FreqDist(alice)
>>> v1000 = list(vocab)[:1000]
>>> mapping = nltk.defaultdict(lambda: 'UNK')
>>> for v in v1000:
...
mapping[v] = v
...
>>> alice2 = [mapping[v] for v in alice]
>>> alice2[:100]
['UNK', 'Alice', "'", 's', 'Adventures', 'in',
'Wonderland', 'by', 'UNK', 'UNK',
'UNK', 'UNK', 'CHAPTER', 'I', '.', 'UNK', 'the',
'Rabbit', '-', 'UNK', 'Alice',
'was', 'beginning', 'to', 'get', 'very', 'tired', 'of',
'sitting', 'by', 'her',
Spot check
• Look at the previous example
• For each line of the code, determine
exactly what it does. Think first, then
insert print commands to show each
result.
Incrementally updating a
dictionary
•
•
•
•
Initialize an empty defaultdict
Process each part of speech tag in the text
If it has not been seen before, it will have zero count
Each time the tag is seen, increment its counter
• itemgetter(n) returns a function that can be called
on some other sequence object to obtain the nth
element
>>> counts = nltk.defaultdict(int)
>>> from nltk.corpus import brown
>>> for (word, tag) in brown.tagged_words(categories='news'):
... counts[tag] += 1
...
>>> counts['N']
22226
>>> list(counts)
['FW', 'DET', 'WH', "''", 'VBZ', 'VB+PPO', "'", ')', 'ADJ', 'PRO', '*', '-', ...]
>>> from operator import itemgetter
>>> sorted(counts.items(), key=itemgetter(1), reverse=True)
[('N', 22226), ('P', 10845), ('DET', 10648), ('NP', 8336), ('V', 7313), ...]
>>> [t for t, c in sorted(counts.items(), key=itemgetter(1), reverse=True)]
['N', 'P', 'DET', 'NP', 'V', 'ADJ', ',', '.', 'CNJ', 'PRO', 'ADV', 'VD', ...]
Look at the parameters of sorted. What does each represent?
Another pattern for updating
import nltk
last_letters = nltk.defaultdict(list)
words = nltk.corpus.words.words('en')
for word in words:
key = word[-2:]
last_letters[key].append(word)
print last_letters['ly']
['abactinally', 'abandonedly', 'abasedly',
'abashedly', 'abashlessly', 'abbreviately',
'abdominally', 'abhorrently', 'abidingly',
'abiogenetically', 'abiologically', ...]
Note that each entry in the dictionary has a unique key
The value part of the entry is a list
index
• Because accumulating a list of words is so
common, NLTK defines a defaultdict(list)
– nltk.Index
>>> anagrams = nltk.Index((''.join(sorted(w)), w) for w
in words)
>>> anagrams['aeilnrt']
['entrail', 'latrine', 'ratline', 'reliant', 'retinal',
'trenail']
What does this do?
Exactly the same as this:
>>> anagrams = nltk.defaultdict(list)
>>> for word in words:
...
key = ''.join(sorted(word))
...
anagrams[key].append(word)
...
>>> anagrams['aeilnrt']
Inverted dictionary
>>> pos = {'colorless': 'ADJ', 'ideas': 'N', 'sleep': 'V', 'furiously': 'ADV'}
>>> pos2 = dict((value, key) for (key, value) in pos.items())
>>> pos2['N']
pos is a dictionary by { … }
'ideas'
pos2 is a dictionary by dict( … )
>>> pos
{'furiously': 'ADV', 'sleep': 'V', 'ideas': 'N',
'colorless': 'ADJ'}
>>> pos2
{'ADV': 'furiously', 'N': 'ideas', 'ADJ': 'colorless',
'V': 'sleep'}
>>> pos.update({'cats': 'N', 'scratch': 'V', 'peacefully': 'ADV', 'old': 'ADJ'})
>>> pos2 = nltk.defaultdict(list)
>>> for key, value in pos.items():
... pos2[value].append(key)
...
>>> pos2['ADV']
['peacefully', 'furiously']
Automatic tagging
• Default
– Tag each token with the most common tag.
>>> tags = [tag for (word, tag) in brown.tagged_words(categories='news')]
>>> nltk.FreqDist(tags).max()
'NN'
NN (noun) is the most common tag in the given text.
>>> raw = 'I do not like green eggs and ham, I do not like them Sam I am!'
>>> tokens = nltk.word_tokenize(raw)
>>> default_tagger = nltk.DefaultTagger('NN')
>>> default_tagger.tag(tokens)
[('I', 'NN'), ('do', 'NN'), ('not', 'NN'), ('like', 'NN'), ('green', 'NN'),
('eggs', 'NN'), ('and', 'NN'), ('ham', 'NN'), (',', 'NN'), ('I', 'NN'),
('do', 'NN'), ('not', 'NN'), ('like', 'NN'), ('them', 'NN'), ('Sam', 'NN'),
('I', 'NN'), ('am', 'NN'), ('!', 'NN')]
NLTK includes an evaluate function for each tagger
>>> default_tagger.evaluate(brown_tagged_sents)
Not very
0.13089484257215028
good!
Regular expression tagger
• Use expected patterns to assign tags
>>> patterns = [
...
(r'.*ing$', 'VBG’),
# gerunds
...
(r'.*ed$', 'VBD'),
# simple past
...
(r'.*es$', 'VBZ'),
# 3rd singular present
...
(r'.*ould$', 'MD'),
# modals
...
(r'.*\'s$', 'NN$'),
# possessive nouns
...
(r'.*s$', 'NNS'),
# plural nouns
...
(r'^-?[0-9]+(.[0-9]+)?$', 'CD'), # cardinal num.
...
(r'.*', 'NN')
# nouns (default)
...
]
Tag applied as soon as a match is found. If no match, defaults to NN
>>> regexp_tagger = nltk.RegexpTagger(patterns)
>>> regexp_tagger.tag(brown_sents[3])
[('``', 'NN'), ('Only', 'NN'), ('a', 'NN'), ('relative', 'NN'), ('handful', 'NN'),
('of', 'NN'), ('such', 'NN'), ('reports', 'NNS'), ('was', 'NNS'), ('received', 'VBD'),
("''", 'NN'), (',', 'NN'), ('the', 'NN'), ('jury', 'NN'), ('said', 'NN'), (',', 'NN'),
('``', 'NN'), ('considering', 'VBG'), ('the', 'NN'), ('widespread', 'NN'), ...]
>>> regexp_tagger.evaluate(brown_tagged_sents)
0.20326391789486245
better
Lookup tagger
• Find the most common words and store
their usual 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)
>>> baseline_tagger.evaluate(brown_tagged_sents)
0.45578495136941344
Look at this carefully
Still better
Refined lookup
• Assign tags to words that are not nouns,
and default others to noun.
>>> baseline_tagger = nltk.UnigramTagger(model=likely_tags,
...
backoff=nltk.DefaultTagger('NN'))
Model size and performance
Evaluation
• Gold standard test data
– Corpus that has been manually annotated
and carefully evaluated.
– Test the tagging technique against the test
case, where the right answers are known. If
it does well there, assume it does well in
general.
N-gram tagging
• Unigram
– Use the most frequent tag for a word
– Must have a “gold standard” for reference
>>> 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)
>>> unigram_tagger.tag(brown_sents[2007])
[('Various', 'JJ'), ('of', 'IN'), ('the', 'AT'), ('apartments', 'NNS'),
('are', 'BER'), ('of', 'IN'), ('the', 'AT'), ('terrace', 'NN'), ('type', 'NN'),
(',', ','), ('being', 'BEG'), ('on', 'IN'), ('the', 'AT'), ('ground', 'NN'),
('floor', 'NN'), ('so', 'QL'), ('that', 'CS'), ('entrance', 'NN'), ('is', 'BEZ'),
('direct', 'JJ'), ('.', '.')]
>>> unigram_tagger.evaluate(brown_tagged_sents)
0.9349006503968017
Testing on the same data as training.
Separate training and testing
If training and evaluation are on the same data, we certainly expect a very
good performance!
More realistically, train on part of the data and test on the rest.
>>> size = int(len(brown_tagged_sents) * 0.9)
>>> size
4160
>>> train_sents = brown_tagged_sents[:size] train on the last 4160 sentences
>>> test_sents = brown_tagged_sents[size:] test on the rest of the sentences
>>> unigram_tagger = nltk.UnigramTagger(train_sents)
>>> unigram_tagger.evaluate(test_sents)
0.81202033290142528
The testing data is now different from the
training data. So, this is a better test of the
process.
Your turn
• Experiment with this tagger.
• Does it matter if you train on the first part or
the last part?
• What is the effect of training on 80% of the
data and testing on the other 20%
• Notice that the training and testing, though on
different sentences, is all from the same
category of the Brown corpus. How well
would you expect the training to do in a
different corpus? If the corpus was also from
a news category? If it was from a novel?
General N-Gram Tagging
• Combine current word and the part of
speech tags of the previous n-1 words to
give the current word some context.
Bigram tagger
>>> bigram_tagger = nltk.BigramTagger(train_sents)
>>> bigram_tagger.tag(brown_sents[2007])
[('Various', 'JJ'), ('of', 'IN'), ('the', 'AT'), ('apartments', 'NNS'),
('are', 'BER'), ('of', 'IN'), ('the', 'AT'), ('terrace', 'NN'),
('type', 'NN'), (',', ','), ('being', 'BEG'), ('on', 'IN'), ('the', 'AT'),
('ground', 'NN'), ('floor', 'NN'), ('so', 'CS'), ('that', 'CS'),
('entrance', 'NN'), ('is', 'BEZ'), ('direct', 'JJ'), ('.', '.')]
This is the
precision – recall
tradeoff of
information
retrieval
>>> unseen_sent = brown_sents[4203]
>>> bigram_tagger.tag(unseen_sent)
[('The', 'AT'), ('population', 'NN'), ('of', 'IN'), ('the', 'AT'), ('Congo', 'NP'),
('is', 'BEZ'), ('13.5', None), ('million', None), (',', None), ('divided', None),
('into', None), ('at', None), ('least', None), ('seven', None), ('major', None),
('``', None), ('culture', None), ('clusters', None), ("''", None), ('and', None),
('innumerable', None), ('tribes', None), ('speaking', None), ('400', None),
('separate', None), ('dialects', None), ('.', None)]
>>> bigram_tagger.evaluate(test_sents)
Reliance on context not seen in training reduces
0.10276088906608193
accuracy.
So, what’s the problem?
• Now, we are matching pairs of words and a
pair is less likely to have occurred before.
• Using context provides greater accuracy,
when it is able to find a match. However,
frequently, it will not find a match.
• Compromise – use the most accurate
tagger for what it can do, then back it up
with another tagger for the parts that do
not get tagged.
Combining taggers
• Use the benefits of several types of
taggers
– Try the bigram tagger
– When it is unable to find a tag, use the
unigram tagger
– If that fails, then use the default tagger
>>> t0 = nltk.DefaultTagger('NN')
>>> t1 = nltk.UnigramTagger(train_sents, backoff=t0)
>>> t2 = nltk.BigramTagger(train_sents, backoff=t1)
>>> t2.evaluate(test_sents)
0.84491179108940495
Spot Check
• Note the order of appearance of the
taggers. Why is that?
• Extend to a trigramtagger, t3, which
backs off to t2.
Assignment for Chapter 5
• Assignment for two weeks:
• NLTK chapter 5: # 3, 12
• Program: (be sure to work alone!)
– Choose one of these: 5.25, 5.30, 5.33