I love deadlines; I especially like the SWOOSHING sound they make as they fly past
— Douglas Adams
Glen has a nice little post on this, and he mentions another favorite of mine: the Monte Carlo simulation to get a handle on schedule risk. The fact is, the statistical math is either not closed form or impractical to evaluate for any real schedule, so the only way to really see what is going on is with a simulation--and it's fast!
The Central Limit Theorem tells us the outcome distribution of a long term schedule with a lot of activities is going to be nearly symmetric--that is, about as many things are going to go right as not over a long term--but the confidence interval, in real numbers, for any particular schedule is only knowable with simulation. And simulation is the best way to get a fast read on the project manager's best friend in statistics: Expected Value
Give it a try!
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