Milestones can be a collection of events (handoffs) that culminate in one milestone. Let’s take the milestone “code freeze” or “code complete.” The code doesn’t magically all become complete on one day; some of the code is completed earlier than others. The code complete/freeze milestone (or more often, code slush) is the last date by which the code is supposed to be complete/frozen. But normally, the code isn’t complete. Some pieces of the code are not yet done, and that implies a delay for the rest of the project.
James noted this phenomenon in his comment: “I often see handoffs that occur on schedule but later turn out to be incomplete, not always easily detectable by the receiving group at the time of the handoff.” That’s why I like to check on handoffs, not only milestones. I’ve seen this happen with all kinds of milestones, but especially with the “code complete” milestone. When the test group starts to test, they realize the milestone hasn’t been met (the code isn’t completely done), even though the schedule has been supposedly met. Eric said, “Each of these milestones also have associated a set of deliverables. In addition, each milestone also has a deliverable called exit criteria so everyone on the team understands what needs to done in order to achieve an exit from the milestone and move on to the next milestone. To me this is a handoff.”
Exit criteria can help make milestones real and accurate. But if the phase exit criteria are not defined, except by the date, or if the PM is only tracking major milestones (because each group is supposed to track their interim milestones), then the project team can miss the milestone, even if the date has occurred. Earned value, if you can calculate it, can help with this. But I find that if I understand the interim handoffs and track them, I know more about the project.
Here’s an example: For projects with several components or several teams working on multiple components, I would expect the designs or code to finish at different times. If I only monitor a “code complete” milestone, I miss all the handoffs between the development groups (which code is ready when and what are the risks of which code is not yet complete?) and the overall handoff of product to the test group. I want to prevent the risk of thinking we’ve met the schedule when we haven’t.