BUILDING NON-NORMATIVE SYSTEMS - THE SEARCH FOR ROBUSTNESS 
AN OVERVIEW 
Mitchell P. Marcus 
Bell Laboratories 
Murray Hill, New Jersey 07974 
Many natural language understanding systems 
behave much like the proverbial high school 
english teacher who simply fails to understand any 
utterance which doesn't conform to that teacher's 
inviolable standard of english usage. But while 
the teacher merely pretends not to understand, our 
systems really don't. 
The teacher consciously stonewalls when 
confronted with non-standard usage to prescribe 
quite rigidly what is acceptable linguistic usage 
and what is not. What is so artificial about this 
behavlour, of course, is that our implicit 
linguistic models are descriptive and not 
prescriptive; they model what we expect, not what 
we demand. People are quite good at understanding 
language which they, when asked, would consider to 
he non-standard in some way or other. 
Our programs, on the other hand, tend to be 
very rigid. They usually fail to degrade 
gracefully when their internal models of syntax, 
semantics or pragmatlcs are violated by user 
input. In essence, the models of linguistic well- 
formedness which these programs embody become 
normative; they prescribe quite rigidly what is 
considered standard linguistic usage and what 
isn't. 
Old solutions to this problem include 
extending a system's linguistic coverage or 
intentionally excluding linguistic constraints 
that are occasionally violated by speakers. But 
neither of these approaches changes the 
fundamental situation - that when confronted with 
input which fails to conform to the system 
builder's expectations, however broad and however 
loose, the system will entirely reject the input. 
Furthermore, these techniques bar a system from 
utilizing the fact that people normally do obey 
certain linguistic standards, even if they violate 
them on occasion. 
More recently, a range of approaches have 
been investigated that allow a system to behave 
more robustly when confronted with input which 
violates its designer's expectations about 
standard english usage. Most of this work has 
been within the realm of syntax. These techniques 
allow grammars to he descriptive without being 
normative. This panel focuses on these techniques 
for building what might be termed non- normative 
systems. Panelists were asked to consider the 
following range of issues: 
Are there different kinds of non-standard 
usage? Candidates for a subclasslficatlon of non- 
standard usage might include the telegraphic 
language of massages and newspaper headlines; the 
informal colloquial use of language, even by 
speakers of the standard dialect; non-standard 
dialects; plain out-and-out grammatical errors; 
and the specialized sublanguage used by experts in 
a given domain. To what extent do these various 
forms have different properties, and are there 
independently characterizable dimensions along 
which they differ? What kinds of generalizations 
can be expressed about each of them individually 
or about non-standard usage in general? 
What are the techiques for dealing with non- 
standard input robustly? A range of techniques 
have been discussed in the literature which can be 
invoked when a system is faced with input which is 
outside the subset of the language that its 
grammar describes. These include~ (a) the use of 
special "un-grammatlcal" rules, which explicitly 
encode facts about non-standard usage; (b) the use 
of "meta-rules" to relax the constraints imposed 
by classes of rules of the grammar; (c) allowing 
flexible interaction between syntax and semantics, 
so that semantics can directly analyze substrlngs 
of syntactic fragments or individual words when 
full syntactic analysis fails. How well do these 
techniques, and others, work with respect to the 
dimensions of non-standard input discussed above? 
What are the relative strengths and weaknesses of 
each of these techniques? 
To what extent are each of these techniques 
useful if one's goal is not to build a system 
which understands input, even if non-standard; but 
rather to build an explicitly normative system 
which can either (i) pinpoint ' grammatical errors, 
or (2) correct errors after pinpointing them? 
(Ironically, a system can be normative in a useful 
way only if it can understand what the user meant 
to say.) 
Are there more general approaches to building 
systems that degrade gracefully that can be 
applied to this set of problems? 
And finally, what the near- and long-term 
prospects for application ~f' ~lese techniques to 
practical working systems? 
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