In Chapter 3 we dealt with words in their own right. We saw that some distinctions can be collapsed using normalization, but we did not make any further generalizations. We looked at the distribution of often, identifying the words that follow it; we noticed that often frequently modifies verbs. We also assumed that you knew that words such as was, called and appears are all verbs, and that you knew that often is an adverb. In fact, we take it for granted that most people have a rough idea about how to group words into different categories.
There is a long tradition of classifying words into categories called parts of speech. These are sometimes also called word classes or lexical categories. Apart from verb and adverb, other familiar examples are noun, preposition, and adjective. One of the notable features of the Brown corpus is that all the words have been tagged for their part-of-speech. Now, instead of just looking at the words that immediately follow often, we can look at the part-of-speech tags (or POS tags). Table 4.1 lists the top eight, ordered by frequency, along with explanations of each tag. As we can see, the majority of words following often are verbs.
Tag | Freq | Example | Comment |
---|---|---|---|
vbn | 61 | burnt, gone | verb: past participle |
vb | 51 | make, achieve | verb: base form |
vbd | 36 | saw, looked | verb: simple past tense |
jj | 30 | ambiguous, acceptable | adjective |
vbz | 24 | sees, goes | verb: third-person singular present |
in | 18 | by, in | preposition |
at | 18 | a, this | article |
, | 16 | , | comma |
The process of classifying words into their parts-of-speech and labeling them accordingly is known as part-of-speech tagging, POS-tagging, or simply tagging. The collection of tags used for a particular task is known as a tag set. Our emphasis in this chapter is on exploiting tags, and tagging text automatically.
Automatic tagging can bring a number of benefits. We have already seen an example of how to exploit tags in corpus analysis — we get a clear understanding of the distribution of often by looking at the tags of adjacent words. Automatic tagging also helps predict the behavior of previously unseen words. For example, if we encounter the word blogging we can probably infer that it is a verb, with the root blog, and likely to occur after forms of the auxiliary to be (e.g. he was blogging). Parts of speech are also used in speech synthesis and recognition. For example, wind/nn, as in the wind blew, is pronounced with a short vowel, whereas wind/vb, as in wind the clock, is pronounced with a long vowel. Other examples can be found where the stress pattern differs depending on whether the word is a noun or a verb, e.g. contest, insult, present, protest, rebel, suspect. Without knowing the part of speech we cannot be sure of pronouncing the word correctly.
In the next section we will see how to access and explore the Brown Corpus. Following this we will take a more in depth look at the linguistics of word classes. The rest of the chapter will deal with automatic tagging: simple taggers, evaluation, n-gram taggers, and the Brill tagger.
Several large corpora, such as the Brown Corpus and portions of the Wall Street Journal, have been tagged for part-of-speech, and we will be able to process this tagged data. Tagged corpus files typically contain text of the following form (this example is from the Brown Corpus):
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 ``/`` 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 ''/'' ./.
By convention in NLTK, a tagged token is represented using a Python tuple. A tuple is just like a list, only it cannot be modified. We can access the components of a tuple using indexing:
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We can create one of these special tuples from the standard string representation of a tagged token, using the function tag2tuple():
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We can construct tagged tokens directly from a string. The first
step is to tokenize the string (using tokenize.whitespace())
to access the individual word/tag strings, and then to convert
each of these into a tuple (using tag2tuple()). We do this in
two ways. The first method, starting at line , initializes
an empty list tagged_words, loops over the word/tag tokens, converts them into
tuples, appends them to tagged_words, and finally displays the result.
The second method, on line
, uses a list comprehension to do
the same work in a way that is not only more compact, but also more readable.
(List comprehensions were introduced in section 3.3.3).
We can also conveniently access tagged corpora directly from Python. The first step is to load the Brown Corpus reader, brown. We then use one of its functions, brown.tagged() to produce a sequence of sentences, where each sentence is a list of tagged words.
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Tagged corpora are available for several other languages. For example, we can access tagged corpora for some Indian languages with nltk_lite.corpora.indian. Figure 4.1 shows the output of the demonstration code indian.demo(). The tags used with these corpora are documented in the README file that comes with this data.
Figure 4.1: POS-Tagged Data from Four Indian Languages
Note
Contributions of tagged data for other languages are invited. These will be incorporated into NLTK.
Linguists recognize several major categories of words in English, such as nouns, verbs, adjectives and determiners. In this section we will discuss the most important categories, namely nouns and verbs.
Nouns generally refer to people, places, things, or concepts, e.g.: woman, Scotland, book, intelligence. Nouns can appear after determiners and adjectives, and can be the subject or object of the verb, as shown in Table 4.2.
Word | After a determiner | Subject of the verb |
---|---|---|
woman | the woman who I saw yesterday ... | the woman sat down |
Scotland | the Scotland I remember as a child ... | Scotland has five million people |
book | the book I bought yesterday ... | this book recounts the colonization of Australia |
intelligence | the intelligence displayed by the child ... | Mary's intelligence impressed her teachers |
Nouns can be classified as common nouns and proper nouns. Proper nouns identify particular individuals or entities, e.g. Moses and Scotland. Common nouns are all the rest. Another distinction exists between count nouns and mass nouns. Count nouns are thought of as distinct entities which can be counted, such as pig (e.g. one pig, two pigs, many pigs). They cannot occur with the word much (i.e. *much pigs). Mass nouns, on the other hand, are not thought of as distinct entities (e.g. sand). They cannot be pluralized, and do not occur with numbers (e.g. *two sands, *many sands). However, they can occur with much (i.e. much sand).
Verbs are words which describe events and actions, e.g. fall, eat in Table 4.3. In the context of a sentence, verbs express a relation involving the referents of one or more noun phrases.
Word | Simple | With modifiers and adjuncts (italicized) |
---|---|---|
fall | Rome fell | Dot com stocks suddenly fell like a stone |
eat | Mice eat cheese | John ate the pizza with gusto |
Verbs can be classified according to the number of arguments (usually noun phrases) that they require. The word fall is intransitive, requiring exactly one argument (the entity which falls). The word eat is transitive, requiring two arguments (the eater and the eaten). Other verbs are more complex; for instance put requires three arguments, the agent doing the putting, the entity being put somewhere, and a location. We will return to this topic when we come to look at grammars and parsing (see Chapter 7).
In the Brown Corpus, verbs have a range of possible tags, e.g.: give/vb (present), gives/vbz (present, 3ps), giving/vbg (present continuous; gerund) gave/vbd (simple past), and given/vbn (past participle). We will discuss these tags in more detail in a later section.
Now that we are able to access tagged corpora, we can write simple simple programs to garner statistics about the tags. In this section we will focus on the nouns and verbs.
What are the 10 most common verbs? We can write a program to find all words tagged with vb, vbz, vbg, vbd or vbn.
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Let's study nouns, and find the most frequent nouns of each noun part-of-speech type. The program in Listing 4.1 finds all tags starting with nn, and provides a few example words for each one. Observe that there are many noun tags; the most important of these have $ for possessive nouns, s for plural nouns (since plural nouns typically end in s), p for proper nouns.
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Listing 4.1 (findtags.py): Program to Find the Most Frequent Noun Tags |
Some tags contain a plus sign; these are compound tags, and are assigned to words that contain two parts normally treated separately. Some tags contain a minus sign; this indicates disjunction [MORE].
The simplest possible tagger assigns the same tag to each token. This may seem to be a rather banal step, but it establishes an important baseline for tagger performance. In order to get the best result, we tag each word with the most likely word. (This kind of tagger is known as a majority class classifier). What then, is the most frequent tag? We can find out using a simple program:
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Now we can create a tagger, called default_tagger, which tags everything as nn.
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Note
The tokenizer is a generator over tokens. We cannot print it directly, but we can convert it to a list for printing, as shown in the above program. Note that we can only use a generator once, but if we save it as a list, the list can be used many times over.
This is a simple algorithm, and it performs poorly when used on its own. On a typical corpus, it will tag only about an eighth of the tokens correctly:
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Default taggers assign their tag to every single word, even words that have never been encountered before. As it happens, most new words are nouns. Thus, default taggers they help to improve the robustness of a language processing system. We will return to them later, in the context of our discussion of backoff.
English nouns can be morphologically complex. For example, words like books and women are plural. Words with the -ness suffix are nouns that have been derived from adjectives, e.g. happiness and illness. The -ment suffix appears on certain nouns derived from verbs, e.g. government and establishment.
English verbs can also be morphologically complex. For instance, the present participle of a verb ends in -ing, and expresses the idea of ongoing, incomplete action (e.g. falling, eating). The -ing suffix also appears on nouns derived from verbs, e.g. the falling of the leaves (this is known as the gerund). In the Brown corpus, these are tagged vbg.
The past participle of a verb often ends in -ed, and expresses the idea of a completed action (e.g. fell, ate). These are tagged vbd.
[MORE: Modal verbs, e.g. would ...]
Common tag sets often capture some morpho-syntactic information; that is, information about the kind of morphological markings which words receive by virtue of their syntactic role. Consider, for example, the selection of distinct grammatical forms of the word go illustrated in the following sentences:
(1a) | Go away! |
(1b) | He sometimes goes to the cafe. |
(1c) | All the cakes have gone. |
(1d) | We went on the excursion. |
Each of these forms — go, goes, gone, and went — is morphologically distinct from the others. Consider the form, goes. This cannot occur in all grammatical contexts, but requires, for instance, a third person singular subject. Thus, the following sentences are ungrammatical.
(2a) | *They sometimes goes to the cafe. |
(2b) | *I sometimes goes to the cafe. |
By contrast, gone is the past participle form; it is required after have (and cannot be replaced in this context by goes), and cannot occur as the main verb of a clause.
(3a) | *All the cakes have goes. |
(3b) | *He sometimes gone to the cafe. |
We can easily imagine a tag set in which the four distinct grammatical forms just discussed were all tagged as vb. Although this would be adequate for some purposes, a more fine-grained tag set will provide useful information about these forms that can be of value to other processors which try to detect syntactic patterns from tag sequences. As we noted at the beginning of this chapter, the Brown tag set does in fact capture these distinctions, as summarized in Table 4.4.
Form | Category | Tag |
---|---|---|
go | base | vb |
goes | 3rd singular present | vbz |
gone | past participle | vbn |
went | simple past | vbd |
These differences between the forms are encoded in their Brown Corpus tags: be/be, being/beg, am/bem, been/ben and was/bedz. This means that an automatic tagger which uses this tag set is in effect carrying out a limited amount of morphological analysis.
Most part-of-speech tag sets make use of the same basic categories, such as noun, verb, adjective, and preposition. However, tag sets differ both in how finely they divide words into categories; and in how they define their categories. For example, is might be just tagged as a verb in one tag set; but as a distinct form of the lexeme BE in another tag set (as in the Brown Corpus). This variation in tag sets is unavoidable, since part-of-speech tags are used in different ways for different tasks. In other words, there is no one 'right way' to assign tags, only more or less useful ways depending on one's goals. More details about the Brown corpus tag set can be found in the Appendix.
The regular expression tagger assigns tags to tokens on the basis of matching patterns. For instance, we might guess that any word ending in ed is the past participle of a verb, and any word ending with 's is a possessive noun. We can express these as a list of regular expressions:
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Note that these are processed in order, and the first one that matches is applied.
Now we can set up a tagger and use it to tag some text.
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How well does this do?
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The regular expression is a catch-all, which tags everything as a noun. This is equivalent to the default tagger (only much less efficient). Instead of re-specifying this as part of the regular expression tagger, is there a way to combine this tagger with the default tagger? We will see how to do this later, under the heading of backoff taggers.
So far the performance of our simple taggers has been disappointing. Before we embark on a process to get 90+% performance, we need to do two more things. First, we need to establish a more principled baseline performance than the default tagger, which was too simplistic, and the regular expression tagger, which was too arbitrary. Second, we need a way to connect multiple taggers together, so that if a more specialized tagger is unable to assign a tag, we can "back off" to a more generalized tagger.
A lot of high-frequency words do not have the nn tag. Let's find some of these words and their tags. The function in Listing 4.2 takes a list of sentences and counts up the words, and returns the n most frequent words. We'll test out this function for the 100 most frequent words:
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Listing 4.2 (wordcounts.py): Program to Find the N Most Frequent Words |
Next, let's inspect the tags that these words have. First we will do this in the most obvious (but highly inefficient) way:
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A much better approach is to set up a dictionary which maps each of the 100 most frequent words to its most likely tag. We can do this by setting up a frequency distribution cfd over the tags, conditioned on each of the frequent words, as shown in Listing 4.3. This gives us, for each word, a count of the frequency of different tags that occur with the word.
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Listing 4.3 (wordtags.py): Program to Find the Most Likely Tags for the Specified Words |
Now for any word that appears in this section of the corpus, we can look up its most likely tag. For example, to find the tag for the word The we can access the corresponding frequency distribution, and ask for its most frequent event:
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Now we can create and evaluate a simple tagger that assigns tags to words based on this table:
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This is surprisingly good; just knowing the tags for the 100 most frequent words enables us to tag nearly half the words correctly! Let's see how it does on some untagged input text:
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Notice that a lot of these words have been assigned a tag of None. That is because they were not among the 100 most frequent words. In these cases we would like to assign the default tag of nn, a process known as backoff.
How do we combine these taggers? We want to use the lookup table first, and if it is unable to assign a tag, then use the default tagger. We do this by specifying the default tagger as an argument to the lookup tagger. The lookup tagger will call the default tagger just in case it can't assign a tag itself.
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We can write a simple (but somewhat inefficient) program to create and evaluate lookup taggers having a range of sizes, as shown in Listing 4.4. We include a backoff tagger that tags everything as a noun. A consequence of using this backoff tagger is that the lookup tagger only has to store word/tag pairs for words other than nouns.
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Listing 4.4 (baseline_tagger.py): Lookup Tagger Performace with Varying Model Size |
Figure 4.2: Lookup Tagger
Observe that performance initially increases rapidly as the model size grows, eventually reaching a plateau, when large increases in model size yield little improvement in performance. (This example used the pylab plotting package; we will return to this later in Section 6.3.4).
Two other important word classes are adjectives and adverbs. Adjectives describe nouns, and can be used as modifiers (e.g. large in the large pizza), or in predicates (e.g. the pizza is large). English adjectives can be morphologically complex (e.g. fallV+ing in the falling stocks). Adverbs modify verbs to specify the time, manner, place or direction of the event described by the verb (e.g. quickly in the stocks fell quickly). Adverbs may also modify adjectives (e.g. really in Mary's teacher was really nice).
English has several categories of closed class words in addition to prepositions, such as articles (also often called determiners) (e.g., the, a), modals (e.g., should, may), and personal pronouns (e.g., she, they). Each dictionary and grammar classifies these words differently.
Part-of-speech tags are closely related to the notion of word class used in syntax. The assumption in linguistics is that every distinct word type will be listed in a lexicon (or dictionary), with information about its pronunciation, syntactic properties and meaning. A key component of the word's properties will be its class. When we carry out a syntactic analysis of an example like fruit flies like a banana, we will look up each word in the lexicon, determine its word class, and then group it into a hierarchy of phrases, as illustrated in the following parse tree.
Syntactic analysis will be dealt with in more detail in Part II. For now, we simply want to make the connection between the labels used in syntactic parse trees and part-of-speech tags. Table 4.5 shows the correspondence:
Word Class Label | Brown Tag | Word Class |
---|---|---|
Det | at | article |
N | nn | noun |
V | vb | verb |
Adj | jj | adjective |
P | in | preposition |
Card | cd | cardinal number |
-- | . | Sentence-ending punctuation |
Now that we have examined word classes in detail, we turn to a more basic question: how do we decide what category a word belongs to in the first place? In general, linguists use three criteria: morphological (or formal); syntactic (or distributional); semantic (or notional). A morphological criterion is one which looks at the internal structure of a word. For example, -ness is a suffix which combines with an adjective to produce a noun. Examples are happy → happiness, ill → illness. So if we encounter a word which ends in -ness, this is very likely to be a noun.
A syntactic criterion refers to the contexts in which a word can occur. For example, assume that we have already determined the category of nouns. Then we might say that a syntactic criterion for an adjective in English is that it can occur immediately before a noun, or immediately following the words be or very. According to these tests, near should be categorized as an adjective:
(4a) | the near window |
(4b) | The end is (very) near. |
A familiar example of a semantic criterion is that a noun is "the name of a person, place or thing". Within modern linguistics, semantic criteria for word classes are treated with suspicion, mainly because they are hard to formalize. Nevertheless, semantic criteria underpin many of our intuitions about word classes, and enable us to make a good guess about the categorization of words in languages that we are unfamiliar with. For example, if we all we know about the Dutch verjaardag is that it means the same as the English word birthday, then we can guess that verjaardag is a noun in Dutch. However, some care is needed: although we might translate zij is vandaag jarig as it's her birthday today, the word jarig is in fact an adjective in Dutch, and has no exact equivalent in English!
All languages acquire new lexical items. A list of words recently added to the Oxford Dictionary of English includes cyberslacker, fatoush, blamestorm, SARS, cantopop, bupkis, noughties, muggle, and robata. Notice that all these new words are nouns, and this is reflected in calling nouns an open class. By contrast, prepositions are regarded as a closed class. That is, there is a limited set of words belonging to the class (e.g., above, along, at, below, beside, between, during, for, from, in, near, on, outside, over, past, through, towards, under, up, with), and membership of the set only changes very gradually over time.
With this background we are now ready to embark on our main task for this chapter, automatically assigning part-of-speech tags to words.
The tag.Unigram() class implements a simple statistical tagging algorithm: for each token, it assigns the tag that is most likely for that particular token. For example, it will assign the tag jj to any occurrence of the word frequent, since frequent is used as an adjective (e.g. a frequent word) more often than it is used as a verb (e.g. I frequent this cafe).
Before a unigram tagger can be used to tag data, it must be trained on a tagged corpus. It uses this corpus to determine which tags are most common for each word. Unigram taggers are trained using the train() method, which takes a tagged corpus:
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Once a unigram tagger has been trained, the tag() method can be used to tag new text:
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The unigram tagger will assign the special tag None to any token that was not encountered in the training data.
Affix taggers are like unigram taggers, except they are trained on word prefixes or suffixes of a specified length. (NB. Here we use prefix and suffix in the string sense, not the morphological sense.) For example, the following tagger will consider suffixes of length 3 (e.g. -ize, -ion), for words having at least 5 characters.
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Earlier we encountered the unigram tagger, which assigns a tag to a word based on the identity of that word. In this section we will look at taggers that exploit a larger amount of context when assigning a tag.
Bigram taggers use two pieces of contextual information for each tagging decision, typically the current word together with the tag of the previous word. Given the context, the tagger assigns the most likely tag. In order to do this, the tagger uses a bigram table, a fragment of which is shown in Table 4.6. Given the tag of the previous word (down the left), and the current word (across the top), it can look up the preferred tag.
tag | ask | Congress | to | increase | grants | to | states |
---|---|---|---|---|---|---|---|
at | nn | ||||||
tl | to | to | |||||
bd | to | nns | to | ||||
md | vb | vb | |||||
vb | np | to | nns | to | nns | ||
np | to | to | |||||
to | vb | vb | |||||
nn | np | to | nn | nns | to | ||
nns | to | to | |||||
in | np | in | in | nns | |||
jj | to | nns | to | nns |
The best way to understand the table is to work through an example. Suppose we are processing the sentence The President will ask Congress to increase grants to states for vocational rehabilitation . and that we have got as far as will/md. We can use the table to simply read off the tags that should be assigned to the remainder of the sentence. When preceded by md, the tagger guesses that ask has the tag vb (italicized in the table). Moving to the next word, we know it is preceded by vb, and looking across this row we see that Congress is assigned the tag np. The process continues through the rest of the sentence. When we encounter the word increase, we correctly assign it the tag vb (unlike the unigram tagger which assigned it nn). However, the bigram tagger mistakenly assigns the infinitival tag to the word to immediately preceding states, and not the preposition tag. This suggests that we may need to consider even more context in order to get the correct tag.
As we have just seen, it may be desirable to look at more than just the preceding word's tag when making a tagging decision. An n-gram tagger is a generalization of a bigram tagger whose context is the current word together with the part-of-speech tags of the n-1: preceding tokens, as shown in the following diagram. It then picks the tag which is most likely for that context. The tag to be chosen, tn, is circled, and the context is shaded in grey. In the example of an n-gram tagger shown in Figure 4.3, we have n=3; that is, we consider the tags of the two preceding words in addition to the current word.
Figure 4.3: Tagger Context
Note
A 1-gram tagger is another term for a unigram tagger: i.e., the context used to tag a token is just the text of the token itself. 2-gram taggers are also called bigram taggers, and 3-gram taggers are called trigram taggers.
The tag.Ngram class uses a tagged training corpus to determine which part-of-speech tag is most likely for each context. Here we see a special case of an n-gram tagger, namely a bigram tagger:
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Once a bigram tagger has been trained, it can be used to tag untagged corpora:
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As with the other taggers, n-gram taggers assign the tag None to any token whose context was not seen during training.
As n gets larger, the specificity of the contexts increases, as does the chance that the data we wish to tag contains contexts that were not present in the training data. This is known as the sparse data problem, and is quite pervasive in NLP. Thus, there is a trade-off between the accuracy and the coverage of our results (and this is related to the precision/recall trade-off in information retrieval.)
Note
n-gram taggers should not consider context that crosses a sentence boundary. Accordingly, NLTK taggers are designed to work with lists of sentences, where each sentence is a list of words. At the start of a sentence, tn-1 and preceding tags are set to None.
One way to address the trade-off between accuracy and coverage is to use the more accurate algorithms when we can, but to fall back on algorithms with wider coverage when necessary. For example, we could combine the results of a bigram tagger, a unigram tagger, and a regexp_tagger, as follows:
Each NLTK tagger other than tag.Default permits a backoff-tagger to be specified. The backoff-tagger may itself have a backoff tagger:
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Note
We specify the backoff tagger when the tagger is initialized, so that training can take advantage of the backing off. Thus, if the bigram tagger would assign the same tag as its unigram backoff tagger in a certain context, the bigram tagger discards the training instance. This keeps the bigram tagger model as small as possible. We can further specify that a tagger needs to see more than one instance of a context in order to retain it, e.g. Bigram(cutoff=2, backoff=t1) will discard contexts which have only been seen once or twice.
As before we test the taggers against unseen data. Here we will use a different segment of the corpus.
>>> accuracy0 = tag.accuracy(t0, brown.tagged('b')) # section b: press-editorial >>> accuracy1 = tag.accuracy(t1, brown.tagged('b')) >>> accuracy2 = tag.accuracy(t2, brown.tagged('b'))
>>> print 'Default Accuracy = %4.1f%%' % (100 * accuracy0) Default Accuracy = 12.5% >>> print 'Unigram Accuracy = %4.1f%%' % (100 * accuracy1) Unigram Accuracy = 81.0% >>> print 'Bigram Accuracy = %4.1f%%' % (100 * accuracy2) Bigram Accuracy = 81.9%
What is the upper limit to tagger performance? Unfortunately perfect tagging is impossible. Consider the case of a trigram tagger. How many cases of part-of-speech ambiguity does it encounter? We can determine the answer to this question empirically:
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Thus, one out of twenty trigrams is ambiguous. Given the current word and the previous two tags, there is more than one tag that could be legitimately assigned to the current word according to the training data. Assuming we always pick the most likely tag in such ambiguous contexts, we can derive an empirical upper bound on the performance of a trigram tagger.
Another way to investigate the performance of a tagger is to study its mistakes. Some tags may be harder than others to assign, and it might be possible to treat them specially by pre- or post-processing the data. A convenient way to look at tagging errors is the confusion matrix. It charts expected tags (the gold standard) against actual tags generated by a tagger:
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Based on such analysis we may decide to modify the tagset. Perhaps a distinction between tags that is difficult to make can be dropped, since it is not important in the context of some larger processing task. We could collapse distinctions with the aid of a dictionary that maps the existing tagset to a simpler tagset:
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We can do this for both the training and test data.
Training a tagger on a large corpus may take several minutes. Instead of training a tagger every time we need one, it is convenient to save a trained tagger in a file for later re-use. Let's save our tagger t2 to a file t2.pkl.
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Now, in a separate Python process, we can load our saved tagger.
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Now let's check that it can be used for tagging.
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[Brief discussion of NLTK's smoothing classes, for another approach to handling unknown words: Lidstone, Laplace, Expected Likelihood, Heldout, Witten-Bell, Good-Turing.]
This chapter has introduced the language processing task known as tagging, with an emphasis on part-of-speech tagging. English word classes and their corresponding tags were introduced. We showed how tagged tokens and tagged corpora can be represented, then discussed a variety of taggers: default tagger, regular expression tagger, unigram tagger and n-gram taggers. We also described some objective evaluation methods. In the process, the reader has been introduced to an important paradigm in language processing, namely language modeling. This paradigm former is extremely general, and we will encounter it again later.
Observe that the tagging process simultaneously collapses distinctions (i.e., lexical identity is usually lost when all personal pronouns are tagged prp), while introducing distinctions and removing ambiguities (e.g. deal tagged as vb or nn). This move facilitates classification and prediction. When we introduce finer distinctions in a tag set, we get better information about linguistic context, but we have to do more work to classify the current token (there are more tags to choose from). Conversely, with fewer distinctions, we have less work to do for classifying the current token, but less information about the context to draw on.
There are several other important approaches to tagging involving Transformation-Based Learning, Markov Modeling, and Finite State Methods. We will discuss these in a later chapter. In Chapter 5 we will see a generalization of tagging called chunking in which a contiguous sequence of words is assigned a single tag.
Part-of-speech tagging is just one kind of tagging, one that does not depend on deep linguistic analysis. There are many other kinds of tagging. Words can be tagged with directives to a speech synthesizer, indicating which words should be emphasized. Words can be tagged with sense numbers, indicating which sense of the word was used. Words can also be tagged with morphological features. Examples of each of these kinds of tags are shown below. For space reasons, we only show the tag for a single word. Note also that the first two examples use XML-style tags, where elements in angle brackets enclose the word that is tagged.
Tagging exhibits several properties that are characteristic of natural language processing. First, tagging involves classification: words have properties; many words share the same property (e.g. cat and dog are both nouns), while some words can have multiple such properties (e.g. wind is a noun and a verb). Second, in tagging, disambiguation occurs via representation: we augment the representation of tokens with part-of-speech tags. Third, training a tagger involves sequence learning from annotated corpora. Finally, tagging uses simple, general, methods such as conditional frequency distributions and transformation-based learning.
We have seen that ambiguity in the training data leads to an upper limit in tagger performance. Sometimes more context will resolve the ambiguity. In other cases however, as noted by Abney (1996), the ambiguity can only resolved with reference to syntax, or to world knowledge. Despite these imperfections, part-of-speech tagging has played a central role in the rise of statistical approaches to natural language processing. In the early 1990s, the surprising accuracy of statistical taggers was a striking demonstration that it was possible to solve one small part of the language understanding problem, namely part-of-speech disambiguation, without reference to deeper sources of linguistic knowledge. Can this idea be pushed further? In Chapter 5, on chunk parsing, we shall see that it can.
Tagging: Jurafsky and Martin, Chapter 8
Brill tagging: Manning and Schutze 361ff; Jurafsky and Martin 307ff
HMM tagging: Manning and Schutze 345ff
Abney, Steven (1996). Tagging and Partial Parsing. In: Ken Church, Steve Young, and Gerrit Bloothooft (eds.), Corpus-Based Methods in Language and Speech. Kluwer Academic Publishers, Dordrecht. http://www.vinartus.net/spa/95a.pdf
Wikipedia: http://en.wikipedia.org/wiki/Part-of-speech_tagging
List of available taggers: http://www-nlp.stanford.edu/links/statnlp.html
Table 4.7 gives a sample of closed class words, following the classification of the Brown Corpus. (Note that part-of-speech tags may be presented as either upper-case or lower-case strings -- the case difference is not significant.)
ap | determiner/pronoun, post-determiner | many other next more last former little several enough most least only very few fewer past same |
at | article | the an no a every th' ever' ye |
cc | conjunction, coordinating | and or but plus & either neither nor yet 'n' and/or minus an' |
cs | conjunction, subordinating | that as after whether before while like because if since for than until so unless though providing once lest till whereas whereupon supposing albeit then |
in | preposition | of in for by considering to on among at through with under into regarding than since despite ... |
md | modal auxiliary | should may might will would must can could shall ought need wilt |
pn | pronoun, nominal | none something everything one anyone nothing nobody everybody everyone anybody anything someone no-one nothin' |
ppl | pronoun, singular, reflexive | itself himself myself yourself herself oneself ownself |
pp$ | determiner, possessive | our its his their my your her out thy mine thine |
pp$$ | pronoun, possessive | ours mine his hers theirs yours |
pps | pronoun, personal, nom, 3rd pers sng | it he she thee |
ppss | pronoun, personal, nom, not 3rd pers sng | they we I you ye thou you'uns |
wdt | WH-determiner | which what whatever whichever |
wps | WH-pronoun, nominative | that who whoever whosoever what whatsoever |
About this document...
This chapter is a draft from Introduction to Natural Language Processing, by Steven Bird, Ewan Klein and Edward Loper, Copyright © 2007 the authors. It is distributed with the Natural Language Toolkit [http://nltk.sourceforge.net], Version 0.7.5, under the terms of the Creative Commons Attribution-ShareAlike License [http://creativecommons.org/licenses/by-sa/2.5/].
This document is Revision: 4518 Wed May 16 20:08:28 EST 2007