'(cached attribute) dffits measure for influence of an observation
based on resid_studentized_internal
uses original results, no nobs loop'

@cache_readonly
def dffits_internal(self):

    hii = self.hat_matrix_diag
    dffits_ = (self.resid_studentized_internal * np.sqrt((hii / (1 - hii))))
    dffits_threshold = (2 * np.sqrt(((self.k_vars * 1.0) / self.nobs)))
    return (dffits_, dffits_threshold)
