Word Sense Disambiguation

Some words have more than one meaning.  The brain seems to have an innate ability to work out what a sentence means.  Take the following two sentences:

"I tied my boat to the bank"

"I put my money in the bank"

In the first sentence the you probably imagine somebody tying their boat to the side of a river, yet in the second sentence you imagine somebody investing their money with a financial institution.  That string of four characters: 'b a n k' has completely changed meaning.

Word sense disambiguation (WSD) is a well researched task in computational linguistics with an important application to lexical simplification.  The majority of previous research splits roughly into three categories:

• Supervised: Using labelled data, a system builds a classifier which can recognise the different senses of a word, from a variety of features in the words surrounding it.
• Unsupervised: With unlabelled data, a system learns the different senses of a word.  Classification of new data makes use of the previously learned senses.
• Knowledge Based: A large knowledge resource such as WordNet provides information about the words which can be used during disambiguation.

 WSD is vital to the task of lexical simplification.  Consider simplifying a sentence from the previous example. If you look up the word 'bank' in a thesaurus you will have a list of synonyms that looks something like the following:

Bank:
Financial Institution; Treasury; Safe;
Edge; Beach; Riverside;

If a system does not employ WSD, then there is no method of telling which of the synonyms are correct for the context.  We do not wish to say "I tied my boat to the treasury", or "I put my money in the riverside".  These examples are at best farcical and at worst nonsensical.  WSD is paramount to selecting the correct set of synonyms.

I will not venture to a full explanation of WSD as applied to lexical simplification.  Suffice to say that there are four papers which I have so far identified as addressing the matter.  These can be found in the lexical simplification list.

• Can Spanish be simpler? LexSiS: Lexical simplification for Spanish. Bott et al. 2012
• Wordnet-based lexical simplification of a document. Thomas and Anderson 2012
• Putting it simply: a context-aware approach to lexical simplification. Biran et al. 2011
• Lexical simplification. De Belder et al. 2010

Orthography, Phonology and Nomenclature. Making sense of word relations!

In my role as computational linguist, I often find myself verging into areas of linguistics which I find nothing short of fascinating.  One such is the complex relation between English orthography (that is, how a word is written) and phonology (how it sounds).  In English, we have a 'deep orthography' meaning that a word doesn't necessarily sound the way it looks, leading to beautiful confusions such as:

weight vs. height

foot vs. food

or (my favourite):

cough vs. enough vs. plough vs. though. vs. through.

That's right, 5 distinct sounds from the letters 'ough'.

We also get the interesting phenomenon that one set of letters, with different pronunciations can have totally different meanings.  For example.

He decided to desert the army.
Camels live in the desert.

This is an example of heteronyms.  Heteronyms are different to homonyms which have the same pronounciation and spelling, but a different meaning. These are different again to heterographs, homophones and synonyms.  The table below defines the potential relations between words.  It is taken mostly from this venn diagram

	Meaning	Spelling	Pronunciation
No Relation	Different	Different	Different
Homophone	Different	-	Same
Heterograph	Different	Different	Same
Heteronym	Different	Same	Different
Homonym	Different	Same	Same
Different Spelling	Same	Different	Same
Different Pronunciation	Same	Same	Different
Synonym	Same	Different	Different
Same Word	Same	Same	Same

• No relation: Two word words which are not related in any sense.
• Homophones:  Words which sound the same, but have different meanings.  Further split into the following two categories:
• Heterographs: Homophones with different spellings.  "There", "Their" and "They're" is a classic example.
• Homonyms: Homophones with the same spelling.  E.g. "Right" (direction) vs. "Right" (entitlement).
• Heteronyms: Words that are spelt the same but have a different sound and meaning. E.g. "desert" (leave) vs. "desert" (sahara) as in the above example.
• Different Spelling: No technical word here, just words which mean and sound the same but are spelt differently. e.g. "Labor" (US spelling) vs. "Labour" (British Spelling).
• Different Pronunciation: Again, no technical word, just two words which are written and mean the same, but sound different. E.g. 'the elephant' vs. 'the circus'.  ('the' takes a different sound in each).
• Synonyms: Two words with the same meaning, but different pronunciations and written forms.  e.g. "friend" and "companion". Useful for lexical simplification as synonyms can be ranked according to their simplicity.
• Same Word: No difference here whatsoever.

So there you have it.  I hope this is a helpful contribution to the often confusing world of word relation nomenclature.  I am certainly much more clear on the distinction between these terms as a result of writing this blog.