'(array) The variance / covariance matrix. Computed using the outer
product of gradients method.'

@cache_readonly
def cov_params_opg(self):

    score_obs = self.model.score_obs(self.params, transformed=True).T
    (cov_params, singular_values) = pinv_extended(np.inner(score_obs, score_obs))
    if (self._rank is None):
        self._rank = np.linalg.matrix_rank(np.diag(singular_values))
    return cov_params
