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Combining Contexts in Lexicon Learning for Semantic Parsing

May 25, 2007

NODALIDA 2007, Tartu, Estonia

Chris BiemannUniversity of Leipzig

Germany

Rainer OsswaldFernUniversität Hagen

Germany

Richard SocherSaarland UniversityGermany

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Outline• Motivation: lexicon extension for semantic parsing

• The semantic lexicon HaGenLex

• Binary features and complex sorts

• Method: bootstrapping via syntactic contexts

• Results

• Discussion

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Motivation• Semantic parsing aims at finding a semantic

representation for a sentence

• Semantic parsing needs as a prerequisite semantic features of words.

• Semantic features are obtained by manually creating lexicon entries (expensive in terms of time and money)

• Given a certain amount of manually created lexicon entries, it might be possible to train a classifier in order to find more entries

• Objective is Precision, Recall is secondary

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HaGenLex: Semantic Lexicon for German

complex sort

size: 22,700 entries of these: 13,000 nouns, 6,700 verbs

WORD SEMANTIC CLASSAggressivität nonment-dyn-abs-situationAgonie nonment-stat-abs-situationAgrarprodukt nat-discreteÄgypter human-objectAhn human-objectAhndung nonment-dyn-abs-situationÄhnlichkeit relationAirbag nonax-mov-art-discreteAirbus mov-nonanimate-con-potagAirport art-con-geogrAjatollah human-objectAkademiker human-objectAkademisierung nonment-dyn-abs-situationAkkordeon nonax-mov-art-discreteAkkreditierung nonment-dyn-abs-situationAkku ax-mov-art-discreteAkquisition nonment-dyn-abs-situationAkrobat human-object... ...

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Characteristics of complex sorts in HaGenLex

In total, 50 complex sorts for nouns are constructed from allowed combinations of:

• 16 semantic features (binary), e.g. HUMAN+, ARTIFICIAL- • 17 sorts (binary), e.g. concrete, abstract-situation...

sort (hierarchy)

semantic features

complex sorts

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Application: WOCADI-Parser

„Welche Bücher von Peter Jackson über Expertensysteme wurden bei Addison-Wesley seit 1985 veröffentlicht?“

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General Methodology

Distributional Hypothesis projected on syntactic-semantic contexts for nouns: nouns of similar complex sort are found in similar contexts

We use three kinds of context elements• Adjective Modifier• Verb-Subject (deep)• Verb-Object (deep)

as assigned by the WOCADI parser for training 33 binary classifiers.

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DataCorpus:• 3,068,945 sentences obtained from the Leipzig Corpora

Collection• parser coverage: 42%• verb-deep-subject relations: 430,916• verb-deep-object relations: 408,699• adjective-noun relations: 450,184

Lexicon• 11,100 noun entries• lexicon extension: 10-fold cross validation on known nouns• Also unknown nouns will be classified

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Algorithm:

Initialize the training set;As long as new nouns get classified { calculate class probabilities for each context element; for all yet unclassified nouns n { Multiply class probs of context elements class-wise; Assign the class with highest probabilities to noun n; }}

Class probabilities per context element:a) count number of per classb) normalize on total number of class wrt. noun classesc) normalize to row sum=1

A threshold regulates the minimum number of different context elements a noun co-occurs with in order to be classified

Bootstrapping Mechanism

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From binary classes to complex sorts• Binary classifiers for single features for all three context

element types are combined into one feature assignment:– Lenient: voting– Strict: all classifiers for different context types agree

• Combining the outcome: safe choices

ANIMAL +/-ANIMATE +/-ARTIF +/-AXIAL +/-... (16 features)

... (17 sorts)

ab +/-abs +/-ad +/-as +/-

Selection:compatible complex

sorts that are minimal w.r.t hierarchy and unambiguous.

result classor

reject

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Results: binary classes for different context types

=5

=1

most of the binary features are highly biased

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Combination of context types =1

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Results for complex sorts=5 =1

Complex sorts with highest

training frequency

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Typical mistakesPflanze (plant) animal-object instead of plant-objectzart, fleischfressend, fressend, verändert, genmanipuliert, transgen, exotisch, selten, giftig, stinkend,

wachsend...

Nachwuchs (offspring) human-object instead of animal-objectwissenschaftlich, qualifiziert, akademisch, eigen, talentiert, weiblich, hoffnungsvoll, geeignet, begabt,

journalistisch...

Café (café) art-con-geogr instead of nonmov-art-discrete (cf. Restaurant)Wiener, klein, türkisch, kurdisch, romanisch, cyber, philosophisch, besucht, traditionsreich, schnieke,

gutbesucht, ...

Neger (negro) animal-object instead of human-objectweiß, dreckig, gefangen, faul, alt, schwarz, nackt, lieb, gut, brav

but:

Skinhead (skinhead) human-object (ok){16,17,18,19,20,21,22,23,30}ährig, gleichaltrig, zusammengeprügelt, rechtsradikal, brutal

In most cases the wrong class is semantically close. Evaluation metrics did not account for that.

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Discussion of ResultsBinary features:• Precision >98% for most binary features• Assigning the smaller class is hard for bias>0.9

Context types• verb-subject and verb-object are better than adjective• verb-subject is best single context for complex sorts • combination always helps for binary features

Complex sorts• Todo: more lenient combination procedure to increase

recall

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Conclusion

• Method for semantic lexicon extension• High precision for binary semantic features• Unknown nouns:

– For 3,755 nouns not in the lexicon, a total of 125,491 binary features was assigned.

– For 1,041 unknown nouns, a complex sort was assigned

• Combination to complex sorts yet to be improved• Combination of different context types improves

results

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Any Questions?

Thank you very much!