Collocation Extraction Based on Modifiability Statistics

Collocation Extraction Based on Modifiability Statistics Joachim Wermter Udo Hahn Computerlinguistik, Friedrich-Schiller-Universit¨at Jena F¨urstengraben 30, D-07743 Jena, Germany wermter@coling.uni-freiburg.de Abstract We introduce a new, linguistically grounded measure of collocativity based on the property of limited modifiability and test it on German PP-verb combinations. We show that our mea- sure not only significantly outperforms the stan- dard lexical association measures typically em- ployed for collocation extraction, but also yields a valuable by-product for the creation of col- location databases, viz. possible structural and lexical attributes. Our approach is language-, structure-, and domain-independent because it only requires some shallow syntactic analysis (e.g., a POS-tagger and a phrase chunker). 1 Introduction Natural language is an open and very flexible com- munication system. Syntax, of course, imposes con- straints, e.g., on word order or the occurrence of par- ticular phrasal types such as PPs or NPs, and lexi- cal semantics imposes, e.g., selectional constraints on conceptually permitted sorts or types within the context of specific verbs or nouns. Nevertheless, natural language speakers usually enjoy an enor- mous degree of freedom to express the content they want to convey in a great variety of linguistic forms. There is, however, a significant subset of expres- sions which do not share this rather free combinabil- ity, so-called collocations. From a linguistic per- spective, they can be characterized by at least three recurrent and prominent properties (Manning and Sch¨utze, 1999): � Non-(or limited) compositionality. The mean- ing of a collocation is not a straightforward composition of the meanings of its parts. For example, the meaning of ‘red tape’ is com- pletely different from the meaning of its com- ponents. � Non-(or limited) substitutability. The parts of a collocation cannot be substituted by seman- tically similar words. Thus, ‘gut’ in ‘to spill gut’ cannot be substituted by ‘intestine’ (see also Lin (1999)). � Non-(or limited) modifiability. Many collo- cations cannot be supplemented by additional lexical material. For example, the noun in ‘to kick the bucket’ cannot be modified as ‘to kick the � holey/plastic/water � bucket’. Considering these observations, from a natu- ral language processing perspective, collocations should not enter, e.g., the standard syntax-semantics pipeline so as to prevent compositional semantic readings of expressions for which this is absolutely not desired. Hence, collocations need to be identi- fied as such and subsequently be blocked, e.g., from compositional semantic interpretation. In computational linguistics, a wide variety of lexical association measures have been employed for the task of (semi-)automatic collocation identifi- cation and extraction. Almost all of these measures can be grouped into one of the following three cate- gories: � frequency-based measures (e.g., based on ab- solute and relative co-occurrence frequencies) � information-theoretic measures (e.g., mutual information, entropy) � statistical measures (e.g., chi-square, t-test, log-likelihood, Dice’s coefficient) The corresponding metrics have been extensively discussed in the literature both in terms of their mathematical properties (Dunning, 1993; Manning and Sch¨utze, 1999) and their suitability for the task of collocation extraction (see Evert and Krenn (2001) and Krenn and Evert (2001) for …

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