Eckhard Bick - VISL

wide coverage tagging and parsing tasks, as in the case of the GPSG based Alvey
Natural Language Tools - ANLT - (Phillips & Thompson, 1987) or the ongoing TOSCA
project (Oostdijk, 1991) using extended affix grammar. Since an existing CFG can be
enhanced by probabilistic indexing of its production rules (cp. 3.5.2), hybrid systems
may be one way to solve the recalcitrant problem of huge parse forests for long
sentences, conceptually inherent to the constituent analysis approach. In (Wauschkuhn,
1996) a chart parser is used to implement 615 PSG rules for German, where every rule
is assigned a "safety factor" measuring "usage plausability". The default for terminal
productions is 1. The safety factor, though seemingly assigned by hand, works much the
same way as rule probabilities in PCFGs, allowing to compute a ranking for every tree
in the parse forest: here, the safety factor of the left side (non-terminal) of a production
is the product of the safety factors of all right hand side symbols. Wauschkuhn's parsing
system assigns complete analyses to 56.5% of the sentences in a 1.6 million word news
text corpus, and partial analyses to 85.7%. Due to the lack of a benchmark corpus, no
correctness rate is given 98 . In contrast to many other systems, a sentence is analysed in
two steps: more than half the rules treat macrostructure (clause-trees), and the rest then
parses each subclause's microstructure individually. Thus, even partial analyses still
construct clause-trees, with less than a fifth of partial analyses exhibiting microstructure
failures in more than one subclause. This additional robustness is reminiscent of
Constraint Grammar, where all rules in principle are perceived as independent of each
other, and most of the structure of a sentence will survive a locally wrong function tag
or a wrong dependency marking.
3.5.3 Constraint Grammar: the holographic picture
(addressing ambiguity directly)
Most words in natural language texts are - seen in isolation - ambiguous with regard to
word class, inflexion, syntactic function, semantic content etc. It is, above all, sentence
context (besides content coherence and the reader's "knowledge about the world") that
determines how a word is to be understood. Constraint Grammar (CG), introduced by
Fred Karlsson (1990) and shaped by the Helsinki School (cp. Karlsson et.al., 1995), is a
grammatical approach that aims at performing such disambiguation by establishing
rules for which of a word form's possible readings is to be chosen, and which readings
are to be discarded in a given sentence context. In the parser itself these rules are
98 Wauschkuhn did experiment with ambiguity (ibd., p. 366), reducing parse forest size by running input text through a PoS
tagger first, but blames the available taggers' high error rate (3-5%) for a corresponding drop in parse quality. The
interesting question is how the experiment would have worked with input from a Constraint Grammar tagger, since such
taggers usually claim much lower error rates than probabilistic systems, cp. (Karlsson et. al., 1995) and (Bick, 1996).
- 146 -