Treebanks and Parsing

1
Treebanks and Parsing

• Jan Hajic
• Institute of Formal and Applied Linguistics
• School of Computer Science
• Faculty of Mathematics and Physics
• Charles University, Prague
• Czech Republic

2
Questions Covered

• Q1 What do you care building a parser?
• Q2 What works, what doesnt?
• Q3 What info is useful, what not?
• Q4 How does grammar writing interact with
  treebank building (TB)?
• Q5 Methodological lessons learned from TB?
• Q6 (Dis)advantages of pre-parsing for TB?
• Q7 Phrase-structure vs. dependency?

3
Q1 What do we really care about building a parser

• What will its output used for
• Deep (semantic structure) parsing
• Translation
• Question answering
• etc.
• Conversion of annotation into features
• Locality good (with todays parsers)
• Accuracy
• Size, speed, (the practical things)

4
Q3What info is useful

• Hard to say
• MST (McDonald), Collins, Charniak surface syntax
  parsers (Czech)
• No function tags used
• Reduced tagset (1100 -gt 43)
• Hand-made reduction worked best! (POS, case if
  possible)
• Lemmatization, word forms used
• Empty categories, co-indexation not used (not
  present)
• Adjunct/argument distinction not used
• Subcat frames not used (not present)

5
Q5Lessons learned

• ! For parsing only !
• Separated surface and deep annotation is good
• Even then, Czech parsing lags behind English
• 1 million word treebank is far from enough
• for languages with rich inflection, that is
• Need for tagset reduction
• Local information helps
• Often can be extracted automatically from the
  annotated treebank
• Lexicalized PS/dependency
• not much difference (so far)

6
Q6 (Dis)advantages of pre-parsing (surface)

• Speed
• Up to 50 faster (100 increase in throughput)
• therefore cheaper
• Consistency better
• Labeling
• Color codes for uncertainty of label assignment
• Disadvantage
• strange errors
• Can be checked for automatically with
  cross-checking

7
Q7 Phrase structure vs. dependency

• If
• (phrase structure) has heads marked
• AND (dependency) has tags suitable for phrase
  labels and no non-projectivity
• Then
• essentially the same thing
• Else...
• ?? determining heads branching labels
  projectivization
• Done on Czech Collins parser 98, ACL 99
• Dependency -gt lexicalized PS (parsing) -gt Dep.