4 Software Project Variables
XP suggests a different strategy. Agree as a team – including the customer – on acceptable level of quality. Agree to consider that time and resources are fixed. The only remaining question is that of scope. What will be delivered? The customer will set the priorities for individual features. You will work on them in turn. The software will always be kept in a releasable state.
So given the variables above the customer agrees that the only thing they affect is scope and that sometimes they won’t get all the features in because things change.