February 12, 2014

Mehmet Ali Yatbaz, Ph.D. 2014

Current position: Software Engineer, Facebook, San Francisco. (Linkedin, Publications).
PhD Thesis: Linguistic Category Induction and Tagging Using the Paradigmatic Context Representations with Substitute Words. Koç University Department of Computer Engineering, February 2014. (PDF, Presentation, Keynote)
M.S. Thesis: Stretch: A Feature Weighting Method for The k Nearest Neighbor Algorithms. Koç University Department of Computer Engineering, October 2007. (PDF, Presentation).
Publications: bibtex.php

Linguistic Category Induction and Tagging Using the Paradigmatic Context Representations with Substitute Words

This thesis introduces a new paradigmatic representation of word contexts. Paradigmatic representations of word context are constructed from the potential substitutes of a word, in contrast to syntagmatic representations, which are constructed from the properties of neighboring words. The potential substitutes are calculated by using a statistical language model that is trained on raw text without any annotation or supervision. Thus, each context is represented as a distribution of substitute words. This thesis introduces models with different properties that can incorporate the new paradigmatic representation, and discusses the applications of these models to the tagging task in natural language processing (NLP).

In a standard NLP tagging task, the goal is to build a model in which the input is a sequence of observed words, and the output, depending on the level of supervision, is a sequence of cluster-ids or predefined tags. We define 5 different models with different properties and supervision requirements. The first model ignores the identity of the word, and clusters the substitute distributions without requiring supervision at any level. The second model represents the co-occurrences of words with their substitute words, and thus incorporates the word identity and context information at the same time. To construct the co-occurrence representation, this model discretizes the substitute distribution. The third model uses probabilistic voting to estimate the distribution of tags in a given context. Unlike the first and second models, this model requires the availability of a word-tag dictionary which can provide all possible tags of each given word. The fourth model proposes two extensions to the standard HMM-based tagging models in which both the word identity and the dependence between consecutive tags are taken into consideration. The last one introduces a generative probabilistic model, the noisy channel model, for the taggin tasks in which the word-tag frequencies are available. In this model, each context C is modeled as a distinct channel through which the speaker intends to transmit a particular tag T using a possibly ambiguous word W. To reconstruct the intended message (T), the hearer uses the distribution of possible tags in the given context Pr(TjC) and the possible words that can express each tag Pr(WjT).

The models are applied and analyzed on NLP tagging tasks with different characteristics. The first two models are tested on unsupervised part-of-speech (POS) induction in which the objective is to cluster syntactically similar words into the same group. The probabilistic voting model is tested on the morphological disambiguation of Turkish, with the objective of disambiguating the correct morphological parse of a word, given the available parses. The HMM-based model is applied to the part-of-speech tagging of English, with the objective of determining the correct POS tag of a word, given the available tags. Finally, the last model is tested on the word-sense disambiguation of English, with the objective of determining the correct sense of a word, given the word-sense frequencies.

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