Ambiguity resolution
There are two primary types of ambiguity that an Apertium pipeline can deal with:
- Morphological ambiguity — when a single form has multiple analyses. The solution to this is #Morphological disambiguation.
- Lexical ambiguity — when a single lexical item (lemma+POS+subcategory) has multiple translations. The solution to this is #Lexical selection.
Distinguishing between the two
Morphological ambiguity
Imagine you have a word that has two different tagsets, e.g.
^houses/house<n><pl>/house<v><tv><p3><sg>$ ^this/this<det><dem><sg>/this<prn><dem><sg>$
Normally your analyser will randomly output ("choose") one of these. Now imagine you have a sentence where the wrong analysis is "chosen" (in this case, twice!):
^The/The<det><def>$ ^motel/motel<n><sg>$ ^houses/house<n><pl>$ ^this/this<prn><dem><sg>$ ^dog/dog<n><sg>$^./.<sent>$
In both of these examples, there is a different lemma+POS+subcategory. However, the following is also an example of morphological ambiguity:
^deer/deer<n><sg>/deer<n><pl>$
As with the previous examples, both analyses are correct output from the transducer, but which one is correct in a given context will vary. (The assumption is that one and only one analysis will be correct for a given context.)
The goal of morphological disambiguation is to choose the correct analysis in cases like these based on the surrounding words.
Lexical ambiguity
An example of lexical ambiguity is the following:
- yard (a unit of length)
- yard (an enclosed area)
What's different about these words? Spelling, pronunciation, part of speech? What [different] contexts might each occur in?
Can we decide which translation is right from the surrounding words?
In both cases, these two words would receive the same analysis: ^yard/yard<n><sg>$. There would likely even be only one entry in a transducer that covers both nouns. But in a bilingual dictionary, depending on the target language, you could easily need two different entries to translate these correctly. Without further mechanisms, one of the options would be "chosen" randomly during translation.
To demonstrate this point, let's take an example from Spanish-to-English translation ("Estoy en la estación de tren" = "I'm at the train station"):
^Estar<vblex><pri><p1><sg>/Be<vbser><pri><p1><sg>$ ^en<pr>/in<pr>/at<pr>/on<pr>$ ^el<det><def><f><sg>/the<det><def><f><sg>$ ^estación<n><f><sg>/season<n><sg>/station<n><sg>$ ^de<pr>/of<pr>/from<pr>$ ^tren<n><m><sg>/train<n><sg>$^.<sent>/.<sent>$
Given the output of the bilingual dictionary, this sentence could end up translated as "I'm in the train season". Given the surrounding context, however, it is possilbe to decide the correct translation alternatives.
There are two main scenarios when lexical ambiguity may occur:
- There are two words in Language A that happen to have the same lemma and POS tags
- (kir) жаш → (eng) age; tear (eye water)
- (kir) ат → (eng) horse; name
- (eng) right → (kir) оң (side); туура (correct)
- (eng) lie → (kir) жат (lie down); калп айт (state a falsehood)
- (eng) bat → (kir) жарганат (animal); таякча (stick for hitting a ball)
- There is a semantic distinction in Language B that is not expressed in Language A
- (kir) кол → (eng) hand; arm
- (kir) күн → (eng) sun; day
- (kir) бир → (eng) one; a/an
- (kir) кара → (eng) look at; watch
- (eng) paper → (kir) кагаз (material); доклад (written work)
- (eng) horse → (kir) ат (for riding, working); жылкы (for herding, eating)
- (eng) sister → (kir) сиңди (younger, of female); карындаш (younger, of male); эже (older)
- (eng) move → (kir) жыл/жылдыр (location iv/tv); кыймылда/кыймылдат (movement iv/tv); көч/көчүр (residence, files iv/tv)
- (eng) fall → (kir) түш (drop from a height / fall down); кула (to the side / fall over); жыгыл (due to loss of footing)
