Part-of-speech tagging is the process of assigning unambiguous grammatical categories to words in context. The crux of the problem is that surface forms of words can often be assigned more than one part-of-speech by morphological analysis. For example in English, the word "trap" can be both a singular noun ("a trap") or a verb ("I'll trap it").
This page intends to give an overview of how part-of-speech tagging works in Apertium, primarily within the
apertium-tagger, but giving a short overview of constraints (as in constraint grammar) and restrictions (as in
apertium-tagger) as well.
- See also: Morphological dictionaries
Consider the following sentence in Spanish ("She came to the beach"):
- Vino (noun or verb) a (pr) la (det or prn) playa (noun)
We can see that two out of the four words are ambiguous, "vino", which can be a noun ("wine") or verb ("came") and "la", which can be a determiner ("the") or a pronoun ("her" or "it"). This gives the following possibilities for the disambiguated analysis of the sentence:
- noun, pr, det, noun → Wine to the beach
- verb, pr, det, noun → She came to the beach
- noun, pr, prn, noun → Wine to it beach
- verb, pr, prn, noun → She came to it beach
As can be seen, only one of these interpretations (verb, pr, det, noun) yields the correct translation. So the task of part-of-speech tagging is to select the correct interpretation. There are a number of ways of doing this, involving both linguistically motivated rules (as constraint grammar and the Brill tagger) and statistically based (such as the TnT tagger or the ACOPOST tagger).
The tagger in Apertium (
apertium-tagger) uses a combination of rules and a statistical (hidden Markov) model.
Hidden Markov models
A hidden Markov model (HMM) is a statistical model which consists of a number of hidden states, and a number of observable states. The hidden states correspond to the "correct" set of tags for a given ambiguous sentence, this would be verb, pr, det, noun in the above example.
apertium-tagger, and indeed in many HMM based part-of-speech taggers, the set of observable states corresponds to a set of ambiguity classes. The ambiguity classes of a model are the set of possible ambiguities (often denoted with ). For example, in the above example, these would be (noun | verb) and (det | prn). The preposition "a" and the noun "playa" are unambiguous therefore don't belong to an ambiguity class. These are calculated automatically from the corpus to be used for training.
- Also referred to as "parts-of-speech", e.g. Noun, Verb, Adjective, Adverb, Conjunction, etc.