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Word classes Chris Brew The Ohio State University
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Word senses The problem is that many words have several distinct meanings. Bank “the ground bordering a river” Bank “an establishment for custody of money” In word-sense disambiguation we try to find out which meaning goes with which instance Perhaps surprisingly, this idea of word sense is seriously problematic.
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A more realistic case Slide (n): the slide of a trombone a childrens’ toy The act of sliding A landslide (and its metaphorical uses) A loss in stock value A kind of musical grace note A specific kind of shot in Curling A head-first slide into third base A fracture in a lode resulting in the dislocation or displacement of a portion of it.
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Dictionary definitions One could just say, each dictionary definition is a sense, but if we also want intuitions, we may have to compromise Even if we do this, which dictionary to use?
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Word classes Two potentially conflicting notions Use word classes to predict next word Use word classes to capture semantic commonalities If we use distributional statistics to build classes, what will they be like?
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Distributional clustering Define the properties of a word that one cares about, and give them numerical values. Pull them together into a vector Viewing the vector as a point in space, cluster the words to form classes
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Dimensions of variation What goes into the vector The most important influence How one measures distance between vectors Options include cosine, KL-divergence, information radius Which algorithm to use Exhaustive enumeration of all potential clusters is way too costly, heuristics are needed.
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Things to cluster by Next word (Brown et al) Syntactic relations (Pereira,Tishby) Parallel corpora (Brown et al, Gale et al) Words in window
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Distance measures Euclidean distance Cosine distance (avoids over-dependence on length)
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Algorithms Top-down tree construction (McMahon and Smith) Bottom-up tree construction (Brown et al.) Guided by loss of MI Classical clustering algorithms K-means Hierarchical clustering Ward’s method
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Where to learn more M&S ch 7 (v. good on background Charniak ch 9 and 10 (v. good on algorithms) Manual for the R statistics system, especially the mva module Schulte im Walde’s thesis, our joint papers
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