13. The method of claim 12 , wherein the model fitting parameters are calculated based on the set of modeled impairment correlation terms.
13. The method of claim 12 , wherein the model fitting parameters are calculated based on the set of modeled impairment correlation terms. 14. The method of claim 13 , wherein the model fitting parameters are re-calculated, before the composite impairment correlation term is determined, based on the subset of modeled impairment correlation terms.