Lecture 6. Synonyms and near-synonyms detection

Examples

• Synonyms: Netherlands and Holland, buy and purchase
• Near synonyms: pants, trousers and slacks, mistake and error

Approaches to synonyms and near-synonyms detection

• Thesaurus-based approach
• Distributional semantics
• Context-based approach
• word2vec
• Web search-based approach

Synonyms in WordNet

Given a word, look for synonyms in every synset.

WordNet NLTK interface

In[1]: for i,j in enumerate(wn.synsets('error')):

In[2]: print "Meaning",i, "NLTK ID:", j.name()

In[3]: print "Definition:",j.definition()

In[4]: print "Synonyms:", ", ".join(j.lemma names())

Wordnet Web interface: [1]

Distributional semantics

Exercise 6.1

Calculate PPMI for Table 1.

Exercise 6.2

Input: def.txt or your own text

Output 1: term-context matrix

Output 2: term-term similarity matrix (use cosine similarity)

Output 3: 2D visualization by means of LSA

Hint: use cfd = nltk.ConditionalFreqDist((term, context) for ...) for computing conditional frequency dictionary

Hint: use R for SVD and visualization

word2vec [Mikolov, Chen, Corrado, Dean, 2013]

Very complex machine learning (deep learning) applied to term-context matrices.

There are two regimes:

• CBOW predicts the current word based on the context
• Skip-gram predicts surrounding words given the current word

word2vec project page: [2] demo: [3]

Example: vec(Madrid) - vec(Spain) + vec(France) = vec(Paris)

Context-based approach (1) [Lin, 1998]

Dependency triple [Lin, 1998]

A dependency triple (w, r, w') consists of two words and the grammatical relationship between them in the input sentence.

I have a brown dog: (have subj I), (I subj-of have), (dog obj-of have), (dog adj-mod brown), (brown adj-mod-of dog), (dog det a), (a det-of dog)

||w, r, w'|| — frequency of (w, r, w')

||w, r, * || — total occurrences of w-r relationships

|| *, *, *|| — total number of dependency triples

Mutual information between w, w':

Results: brief(noun) — affidavit 0.13, petition 0.05, memo-randum 0.05, motion 0.05, lawsuit 0.05, depo-sition 0.05, slight 0.05, prospectus 0.04, docu-ment 0.04 paper 0.04

• some sort of dependency parsing is required
• no difference between synonyms and antonyms (win / loose the game)

Web or corpus search approach

Hearst patterns [Hearst, 1998]

Lexico-syntactic patterns to recognize hyponymy:

• such NP as NP, NP and / or NP;
• NP such as NP, NP and / or NP;
• NP, NP or other NP;
• NP, NP and other NP;
• NP, including NP, NP and / or NP;
• NP, especially NP, NP and / or NP;

Text ⇒ pattern ⇒ concordance

To rank concordances: PatternSim [Panchenko, Morozova, Naets]

Concordance ranking:

Input: Terms C, Corpus D

Output: Similarity matrix S(C x C)

K ← extract concord(D);

K_lem ← lemmatize concord(K);

KC ← filter concord(Klem; C);

S ← get extractionfreq(C;K);

S ← rerank(S; C;D);

S ← normalize(S);

return S

Example ranking Efreq-Cfreq:

P(w_i) — frequency of (w_i) P(w_i, w_j) — extraction probability of (w_i,w_j) demo: [4]