penn tree bank 2/n

Mitchell P Marcus et al.
Building a Large Annotated Corpus of English
Table 1
Elimination of lexically recoverable distinctions.
mg
ung
s
/VB
s/VBZ
do/VB
does/ VBZ
sang/ VBD
singing/ VBG
sung/ VBN
be/VB
is/VBZ
was/VBD
being/ VBG
been / VBN
did/VBD
doing/VBG
done/VBN
have/VB
has/ VBZ
had/VBD
having/VBG
had/VBN
    A second example of lexical recoverability concerns those words that can precede
articles in noun phrases. The Brown Corpus assigns a separate tag to pre-qualifiers
(quite, rather, such), pre-quantifiers (all, ha犷many, nary) and both. The Penn Treebank,
on the other hand, assigns all of these words to a single category PDT (predeterminer).
Further examples of lexically recoverable categories are the Brown Corpus categories
PPL (singular reflexive pronoun) and PPLS (plural reflexive pronoun), which we col-
lapse with PRP (personal pronoun), and the Brown Corpus category RN (nominal
adverb), which we collapse with RB (adverb).
    Beyond reducing lexically recoverable distinctions, we also eliminated certain POS
distinctions that are recoverable with reference to syntactic structure. For instance, the
Penn Treebank tagset does not distinguish subject pronouns from object pronouns
even in cases where the distinction is not recoverable from the pronoun's form, as
with you, since the distinction is recoverable on the basis of the pronoun's position
in the parse tree in the parsed version of the corpus. Similarly, the Penn Treebank
tagset conflates subordinating conjunctions with prepositions, tagging both categories
as IN. The distinction between the two categories is not lost, however, since subor-
dinating conjunctions can be recovered as those instances of IN that precede clauses,
whereas prepositions are those instances of IN that precede noun phrases or preposi-
tional phrases. We would like to emphasize that the lexical and syntactic recoverability
inherent in the POS-tagged version of the Penn Treebank corpus allows end users to
employ a much richer tagset than the small one described in Section 2.2 if the need
arises.
2.1.2 Consistency. As noted above, one reason for eliminating a POS tag such as RN
(nominal adverb) is its lexical recoverability. Another important reason for doing so is
consistency. For instance, in the Brown Corpus, the deictic adverbs there and now are
always tagged RB (adverb), whereas their counterparts here and then are inconsistently
tagged as RB (adverb) or RN (nominal adverb)-even in identical syntactic contexts,
such as after a preposition. It is clear that reducing the size of the tagset reduces the
chances of such tagging inconsistencies.
2.1.3 Syntactic Function. A further difference between the Penn Treebank and the
Brown Corpus concerns the significance accorded to syntactic context. In the Brown
Corpus, words tend to be tagged independently of their syntactic function.' For in-
stance, in the phrase the one, one is always tagged as CD (cardinal number), whereas
An important exception is there, which the Brown Corpus tags as EX (existential there) when it is used
as a formal subject and as RB (adverb) when it is used as a locative adverb. In the case of there, we did
not pursue our strategy of tagset reduction to its logical conclusion, which would have implied tagging
existential there as NN (common noun).
315Computational Linguistics
Volume 19, Number 2
in the corresponding plural phrase the ones, ones is always tagged as NNS (plural com-
mon noun), despite the parallel function of one and ones as heads of the noun phrase.
By contrast, since one of the main roles of the tagged version of the Penn Treebank
corpus is to serve as the basis for a bracketed version of the corpus, we encode a
word's syntactic function in its POS tag whenever possible. Thus, one is tagged as NN
(singular common noun) rather than as CD (cardinal number) when it is the head of
a noun phrase. Similarly, while the Brown Corpus tags both as ABX (pre-quantifier,
double conjunction), regardless of whether it functions as a prenominal modifier (both
the boys), a postnominal modifier (the boys both), the head of a noun phrase (both of
the boys) or part of a complex coordinating conjunction (both boys and girls), the Penn
Treebank tags both differently in each of these syntactic contexts-as PDT (predeter-
miner), RB (adverb), NNS (plural common noun) and coordinating conjunction (CC),
respectively.
    There is one case in which our concern with tagging by syntactic function has led
us to bifurcate Brown Corpus categories rather than to collapse them: namely, in the
case of the uninflected form of verbs. Whereas the Brown Corpus tags the bare form
of a verb as VB regardless of whether it occurs in a tensed clause, the Penn Treebank
tagset distinguishes VB (infinitive or imperative) from VBP (non-third person singular
present tense).
2.1.4 Indeterminacy. A final difference between the Penn Treebank tagset and all other
tagsets we are aware of concerns the issue of indeterminacy: both POS ambiguity in
the text and annotator uncertainty. In many cases, POS ambiguity can be resolved with
reference to the linguistic context. So, for instance, in Katharine He户urn's witty line
Grant can be outspoken-but not by anyone 1 know, the presence of the by-phrase forces
us to consider outspoken as the past participle of a transitive derivative of speak-
outspeak-rather than as the adjective outspoken. However, even given explicit criteria
for assigning POS tags to potentially ambiguous words, it is not always possible to
assign a unique tag to a word with confidence. Since a major concern of the Treebank
is to avoid requiring annotators to make arbitrary decisions, we allow words to be
associated with more than one POS tag. Such multiple tagging indicates either that
the word's part of speech simply cannot be decided or that the annotator is unsure
which of the alternative tags is the correct one. In principle, annotators can tag a word
with any number of tags, but in practice, multiple tags are restricted to a small number
of recurring two-tag combinations: JJINN (adjective or noun as prenominal modifier),
JJIVBG (adjective or gerund/present participle), JJIVBN (adjective or past participle),
NNIVBG (noun or gerund), and RBIRP (adverb or particle).
2.2 The POS Tagset
The Penn Treebank tagset is given in Table 2. It contains 36 POS tags and 12 other
tags (for punctuation and currency symbols). A detailed description of the guidelines
governing the use of the tagset is available in Santorini (1990).'
2.3 The POS Tagging Process
The tagged version of the Penn Treebank corpus is produced in two stages, using a
combination of automatic POS assignment and manual correction.
7 In versions of the tagged corpus distributed before November 1992, singular proper nouns, plural
  proper nouns, and personal pronouns were tagged as "NP," "NPS," and "PP," respectively. The current
  tags "NNP," "NNPS," and "PR-P" were introduced in order to avoid confusion with the syntactic tags
  "NP" (noun phrase) and "PP" (prepositional phrase) (see Table 3).
316Mitchell P Marcus et al.
Building a Large Annotated Corpus of English
Table 2
The Penn Treebank POS tagset
1. CC Coordinating conjunction
  2. CD Cardinal number
3. DT Determiner
  4. EX Existential there
5. FW Foreign word
6. IN Preposition/subordinating
              conjunction
7. JJ Adjective
8. JJR Adjective, comparative
9. JJS Adjective, superlative
10. LS List item marker
11. MD Modal
12. NN Noun, singular or mass
13. NNS Noun, plural
14. NNP Proper noun, singular
15. NNPS Proper noun, plural
16. PDT Predeterminer
17. POS Possessive ending
18. PRP Personal pronoun
19. PP$ Possessive pronoun
20. RB Adverb
21. RBR Adverb, comparative
22. RBS Adverb, superlative
23. RP Particle
24. SYM Symbol (mathematical or scientific)
TO
UH
VB
VBD
VBG
VBN
VBP
VBZ
WDT
WP
WP$
WRB
#
$
to
Inte巧ection
Verb, base form
Verb, past tense
Verb, gerund/present
  participle
Verb, past participle
Verb, non-3rd ps. sing. present
Verb, 3rd ps. sing. present
wh-determiner
wh-pronoun
Possessive wh-pronoun
wh-adverb
Pound sign
Dollar sign
Sentence-final punctuation
Comma
Colon, semi-colon
Left bracket character
Right bracket character
Straight double quote
Left open single quote
Left open double quote
Right close single quote
Right close double quote