def get_zeta_i_j_given_separate_counts(self, y_i, y_j):
		'''
		Parameters
		----------
		y_i, np.array(int)
			Arrays of word counts of words occurring in positi ve class
		y_j, np.array(int)

		Returns
		-------
		np.array of z-scores
		'''
		n_i, n_j = y_i.sum(), y_j.sum()
		delta_i_j = (np.log((y_i + 1) / (1. + n_i - y_i))
		             - np.log((y_j + 1) / (1. + n_j - y_j)))
		return delta_i_j