'(cached attribute) covariance ratio between LOOO and original
This uses determinant of the estimate of the parameter covariance
from leave-one-out estimates.
requires leave one out loop for observations'

@cache_readonly
def cov_ratio(self):

    cov_ratio = (self.det_cov_params_not_obsi / np.linalg.det(self.results.cov_params()))
    return cov_ratio
