Components for a semantic textual similarity system

• Focus on word and sentence similarity• Formal side: define similarity in principle

Characterizing word meaning in context

• Given a word in a particular sentence context: Can we characterize its meaning without reference to dictionary senses?

• Why?– For many lemmas, hard to draw sentence

boundaries (-> Kilgarriff, Hanks in lexicography; Kintsch in cognition; Cruse, Tuggy in cognitive linguistics)

Characterizing word meaning in context

• How?– Compute vector space representation for word in

particular sentence context– Read off: contextually appropriate paraphrases

Approaches

• Make clusters that correspond to senses. In given context, compute weighting over clusters / choose cluster (Reisinger & Mooney, Dinu & Lapata)

• One vector per word: mixes senses– Use context to “bend” word vector, adapt it to

given context (Mitchell & Lapata, Erk & Pado, Thater et al)

• Language modeling (Washtell, Moon & Erk)

Using contextualized word vectors

• Part of sentence similarity approach (Reddy et al)

• Paraphrases• Determine inference rule applicability

Viewpoint from vector space approaches to sentence similarity

• Mitchell & Lapata; Clark, Coecke, Sadrzadeh, Grefenstette; Baroni & Zamparelli, Socher et al

• Mostly applied to phrase pairs / sentence pairs with same structure

• Even Socher et al seem to focus on cases with mostly parallel sentence structure

Similarity between sentences of dissimilar structure

• Central: MWE and alternation detection• lemma-specific paraphrases and MWEs:

covered by automatically induced inference rules• alternations: – passivization– John broke the vase / the vase broke

• Principled approach: Graph rewriting system to transform sentence structure (Bar-Haim et al)

A plug for events and SRL

• Central: identifying events & participants about which the sentences speak

• Semantically equivalent sentences talk about the same events

• Hence, SRL, coreference

A plug for events and SRL

• Once events have been identified:– time and date expressions– modals– negation– embedded propositions