Ambiguity Handling: Human vs. Machine

Term Paper, 2006

16 Pages, Grade: 1



0 Introduction

1 The phenomenon of Ambiguity
1.1 Ambiguity: Definition
1.2 Different Types of Ambiguity
1.3 Garden Path Sentences

2 Syntactic Parsing
2.1 The Garden Path Model
2.2 Constraint-based Model of Parsing
2.3 Reanalysis in Sentence Processing

3 Ambiguity Resolution
3.1 The Way Humans Handle Ambiguity
3.2 The Way Machines Handle Ambiguity

4 Conclusion

5 References

0 Introduction

“Ambiguity is pervasive at all levels of analysis. It has been, is, and is likely to remain the key problem in natural language processing.” (Gadzar 1993:161) This statement by Gerald Gadzar expresses the necessity to cope with the challenge of ambiguity resolution. As the phenomenon of ambiguity is widespread in human language, an interesting question would be: How could a machine be able to handle ambiguity while even humans have difficulties in solving such problems?

This paper will first define the phenomenon of ambiguity and explain the different types of it. An interesting aspect will be the effect of garden path sentences. If the aim is to resolve the problem of ambiguous utterances, the analysis of the syntactic structure of a sentence is obligatory. An overview of this process which is termed syntactic parsing will be given. Different parsing approaches will be discussed. A comparison of the garden path model and the constraint-based model will show different possibilities of processing an ambiguous sentence. Furthermore we will see situations where reanalysis is necessary. This will be followed by a comparison between the way humans resolve ambiguity and the ways machines do. In order to find this out the paper will present some ambiguity resolution techniques for computers. Finally a conclusion will sum up the main points.

1 The phenomenon of Ambiguity

Ambiguity has long since been studied in psycholinguistics (Lombardo 1998:298). There are many words and sentences which may have several meanings and therefore represent a problem in language comprehension [INT1].

1.1 Ambiguity: Definition

The linguistic phenomenon where an expression or an utterance can have more than one meaning is called ambiguity [INT1]. An ambiguous sentence can be interpreted in several different ways, as illustrated in the following example:

(1) I saw the man on the hill with the telescope.

The sentence in (1) is ambiguous because it is not clear whether with refers to the seeing, the man or the hill (Newell 1993:61). Linguists try hard to solve the problem of ambiguity in order to prevent serious misunderstandings in communication. Politicians sometimes use the effect of ambiguity for their own advantages.

(2) I oppose taxes which hinder economic growth.

People would probably interpret the utterance in (2) in the way they prefer it and believe that the politician represents their interests. One could either think that he means all kinds of taxes or only those which hinder economic growth. Positively, people also work with ambiguity in literature, poetry or music for useful purposes. That is how a sentence like the following (3) evokes an artistic effect in so far as blue might either stand for a colour or for sadness:

(3) Don’t it make my brown eyes blue.


1.2 Different Types of Ambiguity

There are different types of ambiguity: To begin with lexical ambiguity, one has to differ between polysemy and homonomy. Both phenomena share the feature that their items have several descriptive meanings although their phonetic or written word-form is the same. In the case of homonyms one can speak of lexemes which have the same form but no connection between their meanings (e.g.: race means either a sports event or a human race). By contrast, polysemous items are characterized by related meanings. This can be explained by the fact that one sense has developed from the other sense (e.g.: wing originally means the wing of a bird and can also be used to talk about the wing of an airplane) (Kortmann 2005:204-205). Besides, structural ambiguity describes a phenomenon where the constituents of a sentence can be attached to each other in different ways. The following expression is an example of structural ambiguity:

(1) The police were ordered to stop drinking after midnight.

Here the prepositional phrase after midnight can be either related to ordered to stop drinking, to drinking or to stop drinking (Newell 1993:158.159). Apart from the attachment of prepositional phrases and adverbial phrases there are two more main English constructions of structural ambiguity: Co-ordinate constructions and noun-noun compounds also belong to the three major sources that have this property (Gazdar 1993:159).

Linguists also make a difference between two kinds of syntactic ambiguity which are referred to as local and global ambiguity. Within local ambiguity the uncertainty about the structure of a sentence remains only temporarily because it can immediately be disambiguated by the linguistic context (Newell 1993:158), whereas global ambiguity can only be resolved by the wider context. In this case the utterance remains ambiguous although the whole lexical information is given.

(1) a. When Fred passes the ball, it always gets to its target.

b. When Fred passes, the ball always gets to its target.

For example the utterance (1a) represents local ambiguity in so far as after the noun phrase the ball, the sentence could go on in different ways. An alternative completion of the sentence would be example (1b).

As people are certain about the structure when the sentence is finished, the ambiguity is only temporary. In comparison to this phenomenon there are sentences like example (2), which do not make clear certain circumstances.

(2) The old books and magazines were on the bench.

Although the sentence is complete, the ambiguity concerning whether both the books and the magazines, or only the books, are old, remains unresolved. Therefore this kind of ambiguity is called global or standing (Wingfield: 1998:238).

1.3 Garden Path Sentences

Humans sometimes tend to misinterpret sentences which are grammatically correct but hard to understand. These sentences “lead the hearer down a garden path” because they lead one in the wrong direction and are therefore called garden path sentences (Newell: 1993:62). The reader or the hearer of such a sentence initially concentrates on one meaning of the sentence which in the end, when the sentence is finished, makes no sense. Thus, after recognizing that one’s intuition about the meaning was wrong, one has to reparse the sentence in a different way (Wingfield 1998:239). Consider the following example:

(1) The horse raced past the barn fell.

The initial understanding of the sentence would be that it concerns an active sentence with raced as its main verb. But by reaching the end of it the parser discovers that the word fell does not fit to this interpretation. Therefore one has to go back and reparse the sentence in order to find out that raced past the barn is a relative clause with a passive participle indicating that not the horse but someone else raced the horse and that not raced but fell is the main verb [INT2]. Garden path sentences are not only obstructive but they can also be a useful tool, for example for jokes:

(2) Time flies like an arrow. Fruit flies like a banana.

Furthermore, psycholinguists have often used the garden path effect in order to show how a “language processor is sent into an incorrect parse” [INT3].


Excerpt out of 16 pages


Ambiguity Handling: Human vs. Machine
University of Marburg  (Fremdsprachliche Philologien)
Proseminar Semantics
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ISBN (Book)
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Ambiguity, Handling, Human, Machine
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Stefanie Dietzel (Author), 2006, Ambiguity Handling: Human vs. Machine, Munich, GRIN Verlag,


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