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Minimally Supervised Morphological Analysis by Multimodal Alignment. David Yarowsky and Richard Wicentowski. Introduction. The Algorithm capable of inducing inflectional morphological analyses of regular and highly irregular forms.
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Minimally Supervised Morphological Analysis by Multimodal Alignment David Yarowsky and Richard Wicentowski
Introduction • The Algorithm capable of inducing inflectional morphological analyses of regular and highly irregular forms. • The Algorithm combines four original alignment models based on: • Relative corpus frequency. • Contextual Similarity. • Weighted string similarity. • Incrementally retrained inflectional transduction probabilities.
Lecture’s Subjects • Task definition. • Required and Optional resources. • The Algorithm. • Empirical Evaluation.
Task Definition Consider this task as three steps: • Estimate a probabilistic alignment between inflected forms and root forms. • Train a supervised morphological analysis learner on a weighted subset of these aligned pairs. • Use the result from step 2 to iteratively refine the alignment in step 1.
Example (POS) • Definitions:
Task Definition cont. • The target output of step 1:
Required and Optional resources • For the given language we need: • A table of the inflectional Part of Speech (POS). • A list of the canonical suffixes. • A large text corpus.
Required and Optional resources cont. • A list of the candidate noun, verb and adjective roots (from dictionary), and any rough mechanism for identifying the candidates POS of the remaining vocabulary. (not based on morphological analysis). • A list of the consonants and vowels.
Required and Optional resources cont. • A list of common function words. • A distance/similarity tables generated on previously studied languages. Not essential If available
The Algorithm • Combines four original alignment models: • Alignment by Frequency Similarity. • Alignment by Context Similarity. • Alignment by Weighted Levenshtein Distance. • Alignment by Morphological Transformation Probabilities.
? ? sing sing take singed taked VBD VBD ? sang VBD Lemma Alignment by Frequency Similarity • The motivating dilemma:
Lemma Alignment by Frequency Similarity cont. • This Table is based on relative corpus frequency:
Lemma Alignment by Frequency Similarity cont. • A problem: the true alignments between inflections are unknown in advance. • A simplifying assumption: the frequency ratios between inflections and roots is not significantly different between regular and irregular morphological processes.
Lemma Alignment by Frequency Similarity cont. • Similarity between regular and irregular forms:
Lemma Alignment by Frequency Similarity cont. • The expected frequency should also be estimable from the frequency of any of the other inflectional variants. • VBD/VBG and VBD/VBZ could also be used as estimators.
Lemma Alignment by Context Similarity • Based on contextual similarity of the candidate form. • Computing similarity between vectors of weighted and filtered context features. Clustering inflectional variants of verbs (e.g. sipped, sipping, and sip).
CWsubj(AUX|NEG)*VkeywordDET?CW*CWobj eating the apple Shlomo is Lemma Alignment by Context Similarity cont. • Example:
Lemma Alignment by Weighted Levenshtein Distance • Consider overall stem edit distance. • A cost matrix with initial distance costs: initially set to (0.5,0.6,1.0,0.98)
Lemma Alignment by Morphological Transformation Probabilities The goal is to generalize a mapping function via a generative probabilistic model.
Lemma Alignment by Morphological Transformation Probabilities • Result table:
Lemma Alignment by Morphological Transformation Probabilities cont. <root>+<stem change>+<suffix><inflection> P(inflection | root,suffix,POS)=P(stemchange | root,suffix,POS) unique
Lemma Alignment by Morphological Transformation Probabilities cont. Example:
Lemma Alignment by Morphological Transformation Probabilities cont. Example: P(solidified | solidify, +ed, VBD) = P(yi | solidify, +ed, VBD) ≈ 1P(yi | ify, +ed) + (1-1)( 2P(yi | fy, +ed) + (1-2)( 3P(yi | y, +ed) + (1-3)( 4P(yi | +ed) + (1-4) P(yi) POS can be deleted
Lemma Alignment by Model Combination and the Pigeonhole Principle • No single model is sufficiently effective on its own. • The Frequency, Levenshtein and Context Similarity models retain equal relative weight. • The Morphological Transformation Similarity model increases in relative weight.
Lemma Alignment by Model Combination and the Pigeonhole Principle • Example:
Lemma Alignment by Model Combination and the Pigeonhole Principle cont. • The final alignment is based on the pigeonhole principle. • For a given POS a root shouldn't have more than one inflection norshould multiple inflections in the same POS share the same root.
Empirical Evaluation • Performance: