Generally speaking, program milestones do not have a probabilistic character, which means that the time for the milestone is fixed and invariant. There is no uncertainty from a business value perspective when the milestone occurs. This is particularly so if the milestone is from an external source, like the day and time that an RFP is due in the customer's hands.

We will soon see that this deterministic characteristic does not carry over to the network schedule. Indeed, the idea of the network schedule is to have a series of linked estimates, one three-point estimate for each task, where the three estimated values are the most likely, the most optimistic, and the most pessimistic. Obviously, right from the beginning there is possible conflict between the business and its fixed milestones and the project with its estimated tasks. The differences in the timelines between the business and the project, as we have noted before, is a risk to be managed by the project manager and set onto the project side of the project balance sheet.