Typical choices for this association measure φ are the sum of the TF-IDF scores or an information retrieval relevance scoring function such as BM-25 (Sorg and Cimiano, 2010).
for the case of Tf-IDf, the value of the j-th element of the topic vector is given by: the mapping function can be represented as the product of a Tf-IDf vector of document d multiplied by an W ×N matrix, X, each element of which contains the Tf-IDf value of word i in document b j : for simplicity, we shall assume from this point on that all vectors are already converted to a Tf-IDf or similar numeric vector form.