21. PFI contracts are generally of a long term nature and it is seldom possible to foresee all the changes that may later be required. Contracts therefore need to contain appropriate provisions for dealing with changing requirements. Our report on Managing the Relationship in PFI Projects (42nd Report, Session 2001-02) noted that 55% of the authorities surveyed had already used change procedures to update their contracts. Most of the changes had related to alterations in services, the introduction of new services, and additional works and changes to the design of buildings.