Obviously, positive pivot features tend to occur in positive instances, so the correlations built on positive instances are more reliable than the correlations built on negative instances; and vice versa.
to satisfy this requirement, we proposed the following formula: where P o (w) and P n (w) indicate the probability of word w in the source domain and the target domain respectively: ( ) ( ) where N o (w) and N n (w) is the number of examples with word w in the source domain and the target domain respectively; N o and N n is the number of examples in the source domain and the target domain respectively.