"NATURAL LANGUAGE TEXTS ARE NOT NECESSARILY GRAMMATICAL AND UNAMBIGUOUS 
OR EVEN COMPLETE." 
Lance A. Miller 
Behavioral Sciences and Linguistics Group 
IBM Watson Research Center 
P. O. Box 218 
Yorktown Heights, NY 10598 
The EPISTLE system is being developed in a research 
project for exploring the feasibility of a variety 
of intelligent applications for the processing of 
business and office text (!'Z; the authors of 
are the project workers). Although ultimately 
intended functions include text generation (e.g., 
4), present efforts focus on text analysis: devel- 
oping the capability to take in essentially 
unconstrained business text and to output grammar 
and style critiques, on a sentence by sentence 
basis. 
Briefly, we use a large on-line dictionary and a 
bottom-up parser in connection with an Augmented 
Phrase Structure Grammar (5) to obtain an approxi- 
mately correct structural description of the 
surface text (e.g., we posit no transformations or 
recovery of deleted material to infer underlying 
"deep" structures). In this process we always try 
to force a single parse output, even in the pres- 
ence of true ambiguity. Grammatical critiques are 
provided by having very strong grammar restrictions 
in an initial processing of the sentence; should 
the application of grammar rules fail to lead to 
the identification of a complete, syntactically 
correct, sentence, we then process the material a 
second time, adding other rules which essentially 
relax certain constraints, such as subject-verb 
number agreement, thereby permitting us to recog- 
nize a wide variety of true grammatical errors. 
The stylistic critiques are based on measurements 
of the detailed hierarchical structure descriptions 
produced by the parser, letting us detect a variety 
of stylistic characteristics judged by "experts" to 
be undesirable: too great a distance between 
subject and verb, too much embedding, unbalanced 
subject/predicate size, excessive negation or quan- 
tification, etc. 
The text corpus used for system construction and 
testing is a set of some 400 business letters, 
mostly written by individuals from within various 
organizations to individuals outside those organ- 
izations. These letters, which consist of approxi- 
mately 2300 sentences, were selected from a larger 
collection (about 2000 letters) as being represen- 
tative of the wide variety of styles, tones, 
subject matter, purposes, lengths, factual content, 
and organization-type found in the overall popu- 
lation of business letters. A corpus differing in 
so many of the above features is also heterogeneous 
with respect to syntactic structures -- and there- 
fore with respect to the grammatical capabilities 
that must be incorporated for correct recognition. 
However, it was one thing to be prepared for struc- 
tural diversity; it was quite another thing to be 
faced with the fact that our business letters are 
not some small to moderate subset of grammatical 
phenomena. Rather, they include all of the common 
and most of the arcane constructions one could find 
in, say, Warriner and Griffith (6). For example, 
the very first sentence we tackled was 29 words 
long and began "How nice it was to receive your 
letter complimenting our Manager, Bud Handy, on his 
courtesy • .. : we ran into extraposition, inver- 
sion, infinitive nominalization, gerund phrase, and 
appositive all within the first 13 words! A prima- 
ry consequence of this rich jumble of syntactic 
scree was the frequent annoyance of being stopped 
dead in our processing tracks as our grammar 
revealed itself to be yet once more incomplete. 
But it was not only the incompleteness of the gram- 
mar (for correct sentences) that gave us trouble: 
many words were not recognized, sometimes sentences 
were incomplete, other times they were truly 
ungrammatical (via normal abnormalities of grammar 
or via what appeared to be a rather thoughtless -- 
or at least uninformed -- scattering of apostrophes 
and semicolons within the text) and often we were 
raced not with our desired single parse but with 
many. These then are the situations which cried 
out for techniques either to keep processing going 
or, at least, to keep it alive long enough for it to 
scratch out detailed informative guesses at struc- 
ture on the parsing floor before expiring. 
The techniques for hardiness and robustness which 
we have developed in the two years of implementa- 
tion, and particularly recently, are mostly 
specific to the five trouble situations referred to 
above. For (i) unrecognized words (words not in our 
125K entry on-line dictionary) we check first 
either for initial capitalization or for an inter- 
nal hyphen, presuming a proper name -- noun -- part 
of speech for the former and either noun or adjec- 
tive for the latter. As we improve our dictionary 
processing, to support efficient affix-stripping 
and stem storage, we now plan to hypothesize parts 
of speech based upon, in particular, the outer 
suffixes (e.g., "ly" pretty conclusively estab- 
lishes multi-syllabic words as adverbs). This more 
"intelligent" processing at the part-of-speech 
level is particularly important for avoiding multi- 
ple false parses. 
167 
For the two situations of either (ii) an incomplete 
grammar failing to process a complete grammatical 
sentence, or (iii) an actual incomplete sentence 
(sentence fragment), we are no able to output a 
single "best" structural description when the gram- 
mar can do no more (Jensen and Heidorn, 
forthcoming). This partial structure is "best" in 
the sense that it provides the largest and most 
continuous coverage of the input text string, and 
it also adheres to certain orderings of parts of 
speech and non-terminal constituents. Our experi- 
ence with such structures is that they are quite 
often correct, always better than a "CANNOT PARSE" 
outcome, and appear to be fairly usable for style 
critiquing. In the future we believe more can be 
done with sentence fragments by assuming, first, 
they are simply to be conjoined to some element of 
the previous sentence, or, second, they are an 
elaboration of an immediately preceding element; in 
either case the partial structure output should 
provide sufficient information to "hook" the frag- 
ments in correctly. 
For (iv) truly ungrammatical sentences~ as 
mentioned previously, we introduce a second pass 
with a number of grammatical restrictions relaxed; 
should any complete sentence structure result we 
can determine which relaxations were responsible 
and thereby actually identify the class of ungram- 
maticality. From the point of view of useful 
applications, this is much more of a desirable 
user-oriented function than an internal robust 
recovery procedure. Nonetheless, from the point of 
view of the style critiques at the sentence and 
paragraph levels, this procedure assures the best 
possible starting point, despite "noise" in the 
input text. 
Finally, (v) the situation of multiple parses is 
dealt with by two techniques. The first is the 
deliberate attempt to construct the grammar rules 
such that no more than a single parse can squeeze 
through in most situations; the second is the 
development of a metric which computes a real 
number for each parse, based on its structural 
features, with the decision rule simply being to 
choose the parse with the smallest number (~). 
Our experience with this metric is that it usually 
leads to selection of the best all-around parse; 
such errors as are made would seem to require 
semantic -- and even pragmatic -- information to be 
weighed in the metric, a capability presently 
beyond our means. 
REFERENCES 
i. Miller, Lance A. "Project EPISTLE: A system for 
the automatic analysis of business correspond- 
ence." Proceedings of the First Annual 
National Conference on Artificial 
Intelligence~ Stanford University, August, 
1980, 280-282. 
2. Miller, Lance A., George E. Heidorn, and Karen 
Jensen "Text-Critiquing with the EPISTLE 
System: An Author's Aid to Better Syntax." 
AFIPS Proceedings of the National Computer 
Conference~ Chicago, May 4-7, 1981, 649-655. 
3. Heidorn, George E., Karen Jensen, Lance A. Mill- 
er, Roy J. Byrd, and Martin S. Chodorow "The 
EPISTLE Text-Critiquing System." IBM Systems 
Journal~ to appear Fall, 1982. 
4. Jensen, Karen Computer Generation of Topic 
Paragraphs : Structure and Style". Paper 
presented at the ACL Session of LSA Annual 
Meeting, New York City, December, 1981 (IBM 
Research Report, 1982). 
5. Heidorn, George E. "Augmented Phrase Structure 
Grammars". In B. Nash-Webber and R. Schank 
(Eds.), Theoretical Issues in Natural Language 
Processing, Association for Computational 
Linguistics, 1975. 
6. Warriner, J. E. and F. Griffith English Grammar 
and Composition. New York: Harcourt, Brace and 
World, Inc., 1963. 
7. Heidorn, George E. "Experience with an easily 
computed metric for ranking alternative 
parses". Presentation at the Association for 
Computational Linguistics Meeting, Toronto, 
Canada, June 17, 1982. 
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