Extraction and description of multi-word lexical units in pl W ord N et 3.0 (2016)

Abstract: In this paper, we present methods of extraction of multi-word lexical units (MWLUs) from large text corpora and their description in plWordNet 3.0. MWLUs are filtered from collocations of the structural type Noun+Adjective (NA).

## introduction our focus in this paper are multi-word lexical units (henceforth, mwlus), derived from collocations (automatically extracted from corpora). as in the case of many linguistic terms, there is no agreement among scholars on their common defining criteria. two main approaches are distinguished. the first one treats as collocations all expressions that tend to co-occur in the immediate syntactic neighbourhood @xcite ). this approach is followed by the constructors of corpora (cf. @xcite . the second approach puts the emphasis on the linguistic properties of collocations such as non-compositionality and impossibility of modification and substitution @xcite . in this approach the term collocation is close to the term multiword expression (henceforth, mwe), used in computational linguistics for the linkage of words of the established meaning, analysed as a whole @xcite and to our understanding of the term mwlu. in the present paper we define mwlu by reference to lexical unit (henceforth, lu), a central element of a wordnet @xcite , a whole attributed with meaning and morphosyntactic properties @xcite . thus, mwlu will be an lu, consisting of more than one word and constituting a semantic and morpho-syntactic whole. it is close in spirit to @xcite proposal saying that mwlu is "built from more than one word, associated with a definite meaning somehow stored in one's mental lexicon and immediately retrieved from memory as a whole" @xcite . such a definition forces one to perceive mwlus as having defined structure and semantics which makes the connection "behave like the single individual" @xcite . ## data preparation in the work on extracting mwes, ipi pan corpus 1 and the plwordnet corpus of the wrocław university of technology @xcite ) corpora were used. the extraction was carried out using the set of mwextractor tools, developed for the purposes of the clarin 2 project. mwextractor is a package of tools, which was created for the purposes of the construction of mwlu's network in plwordnet and their syntactic description. it is the part of a bigger infrastructure for aimed for the work with text corpora. the package user has the access to the data cloud, where they record their own corpora (or uses the existing corpora available on the open licence). mwextractor tools package is available on the open licence. sketch engine is a tool for the work with corpora, which allows for the extraction of collocations on the basis of their grammatical relations @xcite . in many respects sketch engine and mwextractor do not differ from each other. for the purposes of the development of 1 http://korpus.pl/ 2 http://clarin-pl.eu/ mwextractor package new statistical measures were implemented, described in this section. those measures, which are compilations or modifications of the known measures, improved extraction results, described in sections 2 and 3. in the first phase, the authors defined initial data (sets of corpora, tagset, wccl's operators describing relations within a collocation @xcite ). in addition, the order of candidates for mwlu can be changed and the continuity of the elements of a collocation does not have to be preserved. the next stage was a dispersion of collocations, through which candidates whose syntactic traits were regarded interesting, are being promoted. in the mwextractor package, apart from available measures that are present in the subject literature, the measures designed for the purposes of the present work and presented in sections 2.1, 2.2, 2.3 were also implemented. ## syntactically non-compositional mwe's automatic evaluation was the first phase of verification of the extracted collocations. we verified syntactic non-compositionality for na-type collocations (noun and a postposed adjective), for which we defined syntactic idiosyncrasies, attesting the stability of the connection (in such a form) in the corpus. based on a statistical analysis, we argue that mwlus syntactic non-compositionality must have the following features: 1. established word order 2. separability. what we understand by the established word order is the ratio of neutral word order (adjective in postposition) occurrence in the corpus to the alternative word order (adjective in preposition). we took the established word order as the main criterion, and if its occurrence was lower than 87.09%, the algorithm suggested abandoning further procedure @xcite . in the case of reaching more than 87.09 % of occurrence, the algorithm tested separability defined as the ratio of occurrence in the word order with the adjective in preposition and postposition divided by at least one other text word to the sum of occurrences in both word orders, but without no text word between elements of the collocation. finally, by using this method we extracted 607 collocations -potential mwlus. from this list, we rejected several proper names and incomplete phrases. the rest of collocations was automatically accepted. ## applications mwlus are collected in the mwe dictionary, in which the following description of candidates is applied: at present, the dictionary contains 45 thousand mwlus, mainly of nouns and bigrams. mwlu's are grouped together according to syntactic schemes described according to the wccl formalism @xcite ). the dictionary is systematically enlarged.