To evaluate the sentiment of some test review, we calculate the perplexity of each model p + and p − with respect to the whole review.
given a probability distribution p, and a review text S composed of K sentences S = s 1 , ..., s K , each sentence s k : 1 ≤ k ≤ K is composed of a sequence of M k words s k = w k 1 , w k 2 , ..., w k M k ; we calculate the perplexity P P p (S) of a model p with respect to text S. It is a very common measure-ment of how well a probability distribution predicts a sample.