'Scores bigrams using chi-square, i.e. phi-sq multiplied by the number
of bigrams, as in Manning and Schutze 5.3.3.'

@classmethod
def chi_sq(cls, n_ii, n_ix_xi_tuple, n_xx):

    (n_ix, n_xi) = n_ix_xi_tuple
    return (n_xx * cls.phi_sq(n_ii, (n_ix, n_xi), n_xx))
