'(array) The QMLE variance / covariance matrix. Computed using the
numerical Hessian as the evaluated hessian.'

@cache_readonly
def cov_params_robust(self):

    cov_opg = self.cov_params_opg
    evaluated_hessian = self.model.hessian(self.params, transformed=True)
    (cov_params, singular_values) = pinv_extended(np.dot(np.dot(evaluated_hessian, cov_opg), evaluated_hessian))
    if (self._rank is None):
        self._rank = np.linalg.matrix_rank(np.diag(singular_values))
    return cov_params
