It is quite possible that the customers would rather have 90 percent of the system's functionality late if they could just have 10 percent of it on time. The corollary of the principle of communicate with customers/users is quite a bit more shocking, but it could very well be the case. Find out!
If you are communicating with your customers, you should be sure you know their priorities. These can easily be recorded in the requirements specification, but the real challenge is to understand the possibly ever shifting priorities. In addition, you must understand the customers' interpretation of "essential," "desirable," and "optional" and prioritize the requirements accordingly. Will they really be happy with a system that satisfies none of the desirable and optional requirements?
Reference:
Gilb, T., "Deadline Pressure: How to Cope with Short Deadlines, Low Budgets, and Insufficient Staffing Levels," Information Processing, Amsterdam: Elsevier Publishers, 1986.
