Extracting LTAGs from Treebanks

1. Extracting LTAGs from Treebanks Fei Xia 04/26/07

2. Q1: How does grammar extraction work?

3. S VP VP NP ADVP VP* V NP ADV draft still Two types of elementary tree in LTAG Initial tree: Auxiliary tree: • Arguments and adjuncts are in different types of elementary trees

4. Y Y* Y* Adjoining operation

5. They still draft policies

6. The treebank tree

7. Step 1: Distinguish head/argument/adjunct

8. S S VP VP NP NP ADVP VP ADVP PRP PRP RB VBP VBP NP NP RB they they still draft NNS NNS draft still policies policies Step 2: Insert additional nodes

9. Step 3: Build elementary trees #3: #1: #2: #3: #4:

10. #1: #2: VP NP NP ADVP VP* PRP NNS RB they still policies #3: S VP NP VBP NP draft Extracted grammar #4:

11. Q2: What info was missing in the source treebank? • Head/argument/adjunct distinction • Use function tags and heuristics • Raising verbs (e.g., seem, appear) vs. other verbs. • He seems to be late • He wants to be late  Need a list of raising verbs in that language • Features, feature equation (e.g., agreement), …

12. Q3: what methodological lessons can be drawn? • The algorithm for extracting LTAGs from treebanks is straightforward. • Some missing information can be “recovered” based on heuristics, others cannot.  The extracted LTAGs are not as rich as the ones built by hand. • Nevertheless, the grammars have been shown to be useful for parsing, SuperTagging, etc.

13. Q4: What are the advantages of a PS or DS treebank? • The original extraction algorithm assumes the input is a PS treebank. • But it can be easily extended if the input is a DS treebank. • Extract tree segments from DS • Run DS PS algorithm on the segments to get elementary trees

14. Q5: Building a treebank for a formalism or building a general treebank? • I prefer the latter because • A general treebank can be used for different formalisms. • Different grammars under the same formalisms can be extracted. • Annotating a general treebank is often easier.