Ambiguity

Ambiguity is a word, phrase, or statement which contains more than one meaning. Ambiguous words or statements lead to vagueness and confusion, and shape the basis for instances of unintentional humor. For instance, it is ambiguous to say “I rode a black horse in red pajamas,” because it may lead us to think the horse was wearing red pajamas. The sentence becomes clear when it is restructured “Wearing red pajamas, I rode a black horse.”

Similarly, same words with different meanings can cause ambiguity e.g. “John took off his trousers by the bank.” It is funny if we confuse one meaning of “bank” which is a building, to another meaning, being “an edge of a river”. Context usually resolves any ambiguity in such cases.

There are two types of ambiguity: Genuine ambiguities, where a sentence really can have two different meanings to an intelligent hearer, and "computer" ambiguities, where the meaning is entirely clear to a hearer but a computer detects more than one meaning. Genuine ambiguity is not a serious problem for NLP problems; it's comparatively rare, and you can't expect computers to do better with natural language than people. Computer ambiguity is a very serious problem; it is extremely common, and it is where computers do much much worse than humans.

Types of ambiguity

1. Lexical ambiguity

Words have multiple meanings.
"I saw a bat."
bat = flying mammal / wooden club?
saw = past tense of "see" / present tense of "saw" (to cut with a saw.)

2. Syntactic ambiguity.

A sentence has multiple parse trees.

Particularly common sources of ambiguity in English are:

Phrase attachment. "Mary ate a salad with spinach from Califonia for lunch on Tuesday."
"with spinach" can attach to "salad" or "ate"

"from California" can attach to "spinach", "salad", or "ate".

"for lunch" can attach to "California", "spinach", "salad", or "ate"

and "on Tuesday" can attach to "lunch", "California", "spinach", "salad" or "ate".

(Crossovers are not allowed, so you cannot both attach "on Tuesday" to "spinach" and attach "for lunch" to salad. Nonetheless there are 42 possible different parse trees.)`

Conjunction. "Mary ate a salad with spinach from Califonia for lunch on Tuesday and Wednesday."
"Wednesday" can be conjoined with salad, spinach, California, lunch, or Tuesday.

Noun group structure English allows long series of nouns to be strung together using the incredibly ambiguous rule NG -> NG NG. E.g. "New York University Martin Luther King Jr. scholarship program projects coordinator Susan Reid". Even taking "New York" "Martin Luther King Jr." and "Susan Reid" to be effectively single elements, this is 8 elements in a row, and has 429 possible parses.

3. Semantic ambiguity.

Even after the syntax and the meanings of the individual words have been resolved, there are two ways of reading the sentence. "Lucy owns a parrot that is larger than a cat", "a parrot" is extenstensionally quantified, "a cat" is either universally quantified or means "typical cats." Other examples:

"The dog is chasing the cat." vs. "The dog has been domesticated for 10,000 years." In the first sentence, "The dog" means to a particular dog; in the second, it means the species "dog".

"John and Mary are married." (To each other? or separately?) Compare "John and Mary got engaged last month. Now, John and Mary are married." vs. "Which of the men at this party are single? John and Jim are married; the rest are all available."

"John kissed his wife, and so did Sam". (Sam kissed John's wife or his own?)

Compare "Amy's car", "Amy's husband", "Amy's greatest fear", "Michaelangelo's David" etc.

4. Anaphoric ambiguity.

A phrase or word refers to something previously mentioned, but there is more than one possibility.

"Margaret invited Susan for a visit, and she gave her a good lunch." (she = Margaret; her = Susan)

"Margaret invited Susan for a visit, but she told her she had to go to work" (she = Susan; her = Margaret.)

"On the train to Boston, George chatted with another passenger. The man turned out to be a professional hockey player." (The man = another passenger).

"Bill told Amy that he had decided to spend a year in Italy to study art."
"That would be his life's work." (That = art)

"After he had done that, he would come back and marry her." (That = spending a year in Italy)

"That was the upshot of his thinking the previous night" (That = deciding)
"That started a four-hour fight." (That = telling Amy)

In many cases, there is no explicit antecedent.

"I went to the hospital, and they told me to go home and rest." (They = the hospital staff.)

Non-literal speech.

"The White House announced today that ..." ("White House" = the Presidents's staff) (Mentonymy)

"The price of tomatoes in Des Moines has gone through the roof" (= increased greatly) Metaphor.

Ellipsis

The omission of words that are needed for grammatical completion, and are "understood". This is very common in speech, less so in writing. E.g. "I am allergic to tomatoes. Also fish." Understood as "I am also allergic to fish" rather than "Also, fish are allergic to tomatoes." "Mozart was born in Salzburg and Beethoven, in Bonn". Understood as "Mozart was born in Salzburg and Beethoven was born in Bonn"

Extended example

A perfectly typical, not contrivedly literary, actual example, from "Nice disguise: Alito's frightening geniality" by Andrew M. Siegel (The New Republic 11/14/05).

If you are a fan of the justices who fought throughout the Rehnquist years to pull the Supreme Court to the right, Alito is a home run --- a strong and consistent conservative with the skill to craft opinions that make radical results appear inevitable and the ability to build trusting professional relationships across ideological lines.

Metaphors: "fought", "pull to the right", "home run", "craft", "build", "across ... lines". (Probably "home run" was the only conscious use of a metaphor.)

Lexical ambiguities: "fan", "strong", "consistent", arguably "conservative", "opinions", "results", "inevitable", "professional". (The line between metaphor and lexical ambiguity is very unclear.)

Syntactic ambiguities: Does "who fought ..." attach to "fan" or "justices"? Does "to the right" attach to "Court", "pull", "years", "fought", "justices" or "fan"? Is "and the ability" conjoined to "opinions" or "the skill" or "conservative"? Does "across ideological lines" attach to "relationships" or "build"? (The last is an example of the phenomenon, not at all rare, of an ambiguity that makes no actual difference; the meaning of either reading is the same.)

Anaphoric ambiguity: Who are the implicit subject and object of "trusting"?

Semantic ambiguity: "the skill ... the ability": Do these denote unique ontological entities? If not, what do they denote?

The hardest part is to find the logical structure, which is, I would argue, "Since Alito is a strong and consistent conservative ... therefore if you are a fan ... then your opinion should be that Alito is a home run." Notice that "your opinion should be" is omitted in the sentence; the linguistic practice of deleting elements and leaving them implicit is known as ellipsis. Notice also that though syntactically "home run" and "strong and consistent conservative" are in apposition, logically they are entirely separate. The author is presenting it as fact that Alito is a strong and consistent conservative with the skill etc. but that Alito is a "home run" is not a fact, it is the presumed opinion of the hypothetical "you".

Example 2

The juiciest prize is to become the face of a luxury brand such as Dior or Burberry. To have any chance, a model must first have magazine shoots under her designer belt. This fact allows fashion magazines to pay peanuts, even for a cover-shoot.

"The beauty business", The Economist, Feb. 11, 2012.

Lexical ambiguity: Every word except "Dior" and "Burberry". Almost half of the words are used with a meaning that is not their most frequent.

Syntactic ambiguity. Where does "for a cover shoot" attach: peanuts, pay, magazines, or allows?

Anaphoric: "chance" of what?

"This fact": Which fact?

Textual structure: How does [the fact that a model must first have magazine shoots under her designer belt] allow [fashion magazines to pay peanuts, even for a cover-shoot]?

Source:

http://cs.nyu.edu/faculty/davise/ai/ambiguity.html

http://literarydevices.net/ambiguity/