The other thing to note - time to completion often varies from the original estimate, when you finally get to figure out what you are actually building. But it doesn't vary equally, in terms of over-spend and under-spend.
You might end up taking twice as long. You might end up taking half the time estimated. Even with a perfectly honest estimate (which is rare) both scenarios are equally likely. So the overall distribution skews towards late delivery - you are as likely to require 2 more days, as you are to save 1.
You might end up taking twice as long. You might end up taking half the time estimated. Even with a perfectly honest estimate (which is rare) both scenarios are equally likely. So the overall distribution skews towards late delivery - you are as likely to require 2 more days, as you are to save 1.