Eckhard Bick - VISL

8
Conclusion:
The advantages of incrementality
8.1 Evaluating the Portuguese parser:
Specific conclusions
In chapter 3.9, I have shown that the Portuguese CG parser achieves correctness
rates on free text of over 99% for morphology/PoS and 96-97% for syntax, which
compares favourably (ch. 3.5) to both PSG-systems - which are not robust and do not
usually run on free text -, and to probabilistic systems, which hover around the 97%
correctness mark for PoS-tagging, and only rarely succeed in analysing even medium
size sentences correctly in their syntactic entirety. I have suggested (following
Chanod & Tapanainen, 1994) that the advantage of the lexicon and rule based CG
approach over a probabilistic approach resides in the possibility of formulating rules
for exceptions, individual lexemes or rare patterns without disturbing the
functionality of the majority cases, and - as opposed to HMM-taggers in particular -
in the frequent use of long range and unbounded context restrictions (cp. rule type
statistics, ch. 3-7-3). On the other hand, I have striven to document, that a CG
grammar’s advantage over another major family of rule based systems, PSGgrammars,
is not limited to the approach immanent robustness of a parser that
expresses syntactic function by tags, and disambiguates rather than generates, but
also can be made visible on the PSG-grammars’ home turf, syntactic tree structures.
Thus, in the Portuguese system, I have incorporated dependency markers on the
clause level (as opposed to only using them on the group level, like in the
“traditional” ENGCG system), and introduced subclause function tags for finite and
non-finite subclauses. Also, as practical proof of the system’s dependency
information content, a compiler and a set of transformation rules were crafted to
transform CG-output into PSG-style syntactic trees.
Within the growing family of CG-based taggers/parsers, the Portuguese
system is the only fully developed parser for a Romance language, so a certain
typological interest is justified in the degree to which the Portuguese system differs
from or resembles other Constraint Grammars. Areas of interest are (a) the notational
system as such, (b) ambiguity and rule set typology, and (c) performance.
At present, Constraint Grammar projects have been launched for a variety of
languages, of which at least 5 (English, Portuguese, Swedish, Norwegian,
Estonian) 248 have published morphological and/or syntactic tag sets. A comparison
shows (below) that at least the Indo-European grammars share large parts of their
248 In my Spanish Constraint Grammar, the morphological and syntactic tag sets are almost identical to the ones used for
Portuguese.
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