Disambiguation of Super Parts of Speech (or Supertags): Almost Parsing

Disambiguation of Super Pint, of Speech (or Supertags) Ahnost Parsing Aravind i(. Joshi and B. Sriniw~s Depa.rtment of Computer and Information Science University o[ Pen nsylvanh~ I>hiladelphia, PA 19104, USA {joshi, stint} ~linc.cis.upeiln.edu Abstract: In a lexicalized grammar Ibrnlal- isni such as LexicMized Tree-Adjoining (h'~unmar (I3'AG), each lexicM item is associated with at least one elementary structure (supertag) that localizes syntactic a.nd semantic dependencies. Thus a parser for a lexicalized grammar must search a large set of supertags to choose the right ones to combine for the parse of the sentence. We present techniques I'or dlsambiguating supertags using local inlorlnlttion s~Lch as lexicM preference and local lexicN dependencies. Tim similarity between LTAG and l)ependency grammars is ex- ploited in the dependency niodel of snpertag dis- a.mbiguation. The performance results for vari- otis models of supert;tg disambigu~ttk)n such as unigram; trigram and dependency-based models are presented. 1 Introduction l>art-of-spee<:h disanll>iguation techni<lues (tag- gers) are often used to eliminat<, (or sul>sl;an- tlally reduce) the lm.rt-of-spee,<;h anil>iguity prior to parsing. The ta.ggel's are all local hi the sense that they use inform~tion front a limited context in deciding which tag(s) to choose for each word. As is well known, these taggers are quil;e, success- ful. In a lexicalized grammar such its the I,exicM- ized "Dee-Adjoining Grammar (13~AG), each lex- ical item is associated with at least one elemen- tary structure (tree). The elementary structures of I'PAG localize dependencies, including long distance dependencies, by requiring that M1 and only the dependent elements be present within the saute structnre. As a result of this localiza- tion, a IoxicM item may be (a.nd, in general, a,1- most alwa,ys is) associa,ted with more. than one elementary structure. ~Ve will cMl these ele.- mentary structures supertags, in order to dis- tinguish them l'rom the standard part-of-speech tags. Note that even when a word has a unique standard part-of-speech, say a verb (V), there will usually lie more than one superta.g associated with this word. Since when the parse is complete, there is only one supertag for each word (assum- ing there is no global ambiguity), an L'['AC, parser (SchMms, 1988) nee.ds to search a large space o1" supertags to select the right one lbr each word before combining them for the parse of a. sen- tence. It is this 1)roblem of supertag dis;unbigua- tion that we address in I, his paper. Since l,'l.'A(',s are lexlcalized, we are. presented with a novel opportunil;y to elimill;tte or substan- tially reduce the supertag assignnmnt ambigu- ity by using local information such a.s local lex- ical dependencies, prior to parsing. As in stan- dard lmrt-ofslieech disambiguatioii, we can use local statistical iufortnatiot~ in the term o[n-gt'anl models based Oil the distril)ution of stiperl;ags hi a I,'I'A(I liarsed corpus. Moreover, since the sli- l)erta.gs elicode depemde, ncy hfl'ornlal;k)n~ we can also use informa.tion about the distribution of dis- tances between a, given superi;ag and its depen- dent su perl;ags. Note that its ill sta,ndard part-of-speech disaun- biguation, superl;ag disambiguation could have been done …

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