Therefore, (Shinnou and Sasaki, 2014) defined P r (x), the prior of x, as follows: , where x denotes a data in the domain R and x has a set of features, that is, They also obtain P R (f i ) using the following equation: .
here, n(R; f ) is the frequency of feature f in the domain R, and n(R) is the number of data in the domain R. the probability density ratio is obtained as follows: 3 Proposed Method The likelihood of the keyword in the target domain is l x , and l x is set as the value of IDF in the target domain of w: here N is the number of articles in the article collection in the target domain, and d i is the number of articles containing the word w in the article collection in the target domain.