'(cached attribute) Cooks distance
uses original results, no nobs loop'

@cache_readonly
def cooks_distance(self):

    hii = self.hat_matrix_diag
    cooks_d2 = ((self.resid_studentized_internal ** 2) / self.k_vars)
    cooks_d2 *= (hii / (1 - hii))
    from scipy import stats
    pvals = stats.f.sf(cooks_d2, self.k_vars, self.results.df_resid)
    return (cooks_d2, pvals)
