We use a 2x2 contingency table of a feature f and a class c to introduce In the table, W denotes the number of documents in the corpus in which feature f and class c cooccur, X the number of documents in which f occurs without c, Y the number of documents in which c occurs without f, and Z the number of documents in which neither c nor f occurs.
n = W + X + Y + Z is the total number of documents in the corpus.