I'm writing my project management book. I have no idea how far along I am. (Wait, I promise to explain.) When I write, I have several phases: the exploratory phase, where I write articles, the write-it-down phase, where I write the whole thing down (in chunks, of course), and the editing phase. I'm in the write-it-down phase right now. In this phase, I need hours at a time to blast away at a chapter until there's little enough of it left so I can write it in spurts. (I hope you understood that.)
I was talking with my editor this morning about when I would be done with the first draft, the write-it-down part. I explained that I had a slim chance of being done by mid-October, and a stronger chance of being done at Thanksgiving. Here's the graph of what that looks like:
Note that there's a 50% chance I'll be done with this phase, the write-it-down phase, by mid October. I have a bunch of writing time between now and then for the book. I have some other writing to do, some web site updates, etc. , but most of my time is available for writing the book. If I miss mid-October, I have no writing time until almost the middle of November. That's why if I'm close in mid-October, I might be able to finish in mid-November. But if I miss mid-October because I'm not close, the mid-November date is quite risky. I have a bunch of writing time from mid-November through early December, but I expect to be traveling again and teaching, which changes my writing time.
This is an example of probabilistic scheduling. I don't even have a 100% completion date (which you might have to have for one of your projects), because I will be replanning in mid-October, no matter what. Any 100% date I give now will be wrong, wrong, wrong. It will either be too optimistic or too pessimistic, and I won't know which one until October.
It doesn't matter what kind of lifecyle you use, the further out the dates are, the less you know. You can use probabilistic scheduling to help you, the project team, and your sponsors see the risk in the schedule.