code: def _at_percentile(tau, var_tau, percentile):
      
      assert (percentile >= 0.0) and (percentile <= 1.0)
      c_val = _scaling(tau, var_tau)
      k_val = _dof(tau, var_tau)
      return np.sqrt(c_val * chi2.ppf(percentile, df=k_val))
docs: Returns the value of the inverse chi-2 distribution at the given
      percentile from the mean and variance of the uncertainty model, as
      reported in equations 5.1 - 5.3 of Al Atik (2015)