Conditional random fields with hidden variables have been studied so far for other tasks.
let us consider how to infer the sentiment polarity p ∈ {+1, −1}, given a subjective sentence w and its dependency tree h. The polarity of the root node (s 0 ) is regarded as the polarity of the whole sentence, and p can be calculated as follows: the polarity of the subjective sentence is obtained as the marginal probability of the root node polarity, by summing the probabilities for all the possible configurations of hidden variables.