To reduce the reliance on external resources, we also experiment with another method based on mutual information (MI) on the source labeled data D s to automatically derive a pseudo sentiment lexicon.
we first extract only adjectives, adverbs and verbs from the documents in D s , and measure each remaining word's MI with the positive and the negative classes: where w i denotes the i th word in V, y ∈ {+, −} is a sentiment label, andp(w i , y) is the empirical probability of observing w i and y together.