6.12 It is not recommended that contract clauses are agreed whereby it is up to either party to the contract to demonstrate whether or not a particular change in costs has been reflected in the indexation agreed in the contract. In practice, it will be extremely difficult to identify in isolation the impact of a single factor on an index which is affected by a multitude of factors. In addition, an increase in a particular area of costs may only be reflected in indexation cumulatively over a period of time which can be difficult to identify.