Re:But what about non-IT projects? (Score 1)

The specify-design-build-test(maybe)-deliver process is deterministic. If it's not, what makes you think you can predict its schedule in the first place? 5% on time may be no more than dumb luck.

Since you clearly now agree that we are speaking of a determenistic process, you also have to agree that Turings Halting Theorm applies. Thus the results of any decision the process demands may not be predictable other than by running the process. The more decisions the process demands, the less predictable they will be. The less predictable the projects path, the less likely you can meet a pre-ordained schedule of any type.

Thus, no matter what the deterministic process is and no matter how dedicated and skilled the people executing it, the probability of meeting any particular schedule is inversly proportional (at least!) to the number of decisions the process demands.

Of course, this assumes that you have customers who:

1. Know exactly what they want
2. Are articulate enough say what they want
3. Won't change their (alleged) mind

...and how often does THAT happen?

No wonder the toy projects in biz school give the pointy-haired types the idea that schedules can be met!