Semantic Role Labelling Using Chunk Sequences Ulrike Baldewein Katrin Erk Sebastian Padó Saarland University Saarbrücken Detlef Prescher Amsterdam University Amsterdam

1. Representation for Classification Usual choice: Classify Constituents  Intuition: one argument  one constituent  Not available in this task Classify words?  Too fine-grained Classify chunks?  Data anylsis: Not always the right level 34% of arguments: more than one chunk 13% of arguments: do not respect chunk boundaries

Chunk sequences as classification instances Sequences of chunks and chunk parts  Adaptive level of structure  „Potential constituents“ [ NP Britain´s] [ NP manufacturing industry] [ VP is transforming] [ NP itself] [ VP to boost] [ NP exports] ARG0 = NP_NP V = VP[VBG] ARG1 = NP ARG2 = VP_NP

Frequency-based chunk sequence filtering Filter 1: Use only sequence types which realise arguments in training set 1089 types  Zipf distributed NP (23,000) S (5,000) NP_PP_NP_PP_NP_VP _PP_NP_NP (1) Filter 2: Use only frequent sequence type (f(s)>10) Examine material between sequence and target  Divider sequences  Also Zipf distributed Empty divider (14,000) NP (10,000)  Similar to „Path“ Filter 3: Use only seq.s with frequent divider (f(d)<10) Filter 4: Use only seq.s co- occurring frequently with some divider (f(s,d)<5)

Results of filtering Leaves 87 sequence types (was 1089)  43,777 tokens in devel set (about 1 seq / word)  8,698 are proper arguments (about 20%) Bad news: representation loses 16% of proper arguments

2. Features „Shallow features“:  Simple properties „Higher-level features“:  Syntactic properties (mostly heuristic) „Divider features“:  Shallow and higher-level properties of dividers

EM-based clustering Measure fit between objects y 1 (pred:arg) and y 2 (sequence)  Example: How well does NP fit give:A1? y 1 and y 2 are independent and generated by cluster  p(y 1,y 2 ) =  c p(c) p(y 1 |c) p(y 2 |c)  EM derives clusters from training data  Intention: Generalisation within clusters Features: e.g. „most likely argument slot for this sequence for this predicate“

3. Procedure Filter sequences from training set Compute features for sequence tokens and their dividers (training + development + test set) Estimate Maximum Entropy model on training set Classify sequences from devel / test set Recover semantic parses

Two-step classification procedure Classifier 1: Argument recognition  Binary decision about argumenthood  All argument classes conflated into ARG Classifier 2: Argument labelling  Consider only sequences assigned ARG by step 1

4. Classification result: Sequence chart Themanwiththebeardsleeps A0 (70%), A1 (20%)NOLABEL (70%), AM-MOD(25%) A0 (60%), NOLABEL (40%) A0 (65%), A1 (25%) Need to find optimal „semantic parse“ of argument labels

Semantic parse recovery Find most probable semantic parse p = (l 1,l 2,...) Step 1: Beam search:  Simple probability model with independence assumption: P bs (l 1,l 2,...) =  i P c (l i ) Step 2: Reestimation  Global considerations: [A0 A0]  Use counts from training set: P(l 1,l 2,...) = P bs (l 1,l 2,...) * P tr (l 1,l 2,...)

5. Results (Development Set) PrecisionRecallF-score Upper Bound Step 1 (ARG only) Final Upper Bound: given by lost chunk sequences But filtering is necessary  Only sequence frequency filtering (filter 1 and 2): Good news: 9% arguments are lost (now 16%) Bad news: 127,000 sequences (now 44,000)  Argument recognition much more difficult  F-score with same features only 0.38

Results Two steps have different profiles  Arg identification: shallow and divider features important  Arg labelling: shallow and higher-level features important Clustering features unsuccessful: Increase precision at cost of recall  Feature „most probable label for sequence“  Successful in SENSEVAL-3 model Largest problem is recall PrecisionRecallF-score Upper Bound Step 1 (ARG only) Final

What I talked about... and more Chunk sequences for SRL  Adaptive representation with „higher-level“ features  Recall problem (Filtering loses proper arguments)  EM-based features promising, but currently not helpful Since submission  Maxent vs. memory-based learner: virtually same result Left to do  Detailed error analysis  More intelligent filtering  Better features