1.37 As PFI contracts are generally of a long term nature, the service requirement which is set out in the contract should take into account not only the authority's current requirements but also its future ones, to the extent that these are identifiable and quantifiable. Over the life of a PFI contract, however, not all possible changes, whether in the authority's requirements or the contractor's methods of delivering those requirements, can be foreseen. Relationships are dynamic so contracts should be capable of change, and change should be accepted as a normal and necessary part of a major programme rather than a sign of failure. The contract should, therefore, contain a mechanism by which changes may be proposed by either party and evaluated and approved prior to implementation. That evaluation should also consider the impact of change on the value for money of the deal, how the change will be priced and how costs and savings should be apportioned.