Do you use business cases to evaluate the performance of your outsourcing decisions?

A recent Dutch survey showed that half of the companies outsourcing do not use a business case as a means to evaluate the performance of their outsourcing. The reason may be that the business case models mostly used do not allow for incorporating flexibility. The typical Discounted Cash Flow (DCF)-based model is very static and does not cope well with change. The world around us is however changing ever faster. How do you face this problem ? Do you in your company use other models, for example Real-Option methodology ?