Yup. If you look at the factor your estimates are off, they'll follow a lognormal distribution.
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And it's natural, if you think about it, that any schedule which doesn't include slack between tasks will almost certainly run long. (Even assuming you've got a mean of 0 -- as many tasks go short as go long -- you're not going to get lucky enough that the tasks that go short do it 'just right' to make up for the tasks that go long.)
So maybe include some slack.
Still, maybe you want to know how often will your schedule run long. So do some measurements of historical data to find out how your task estimates work out in practice.
Plug those in and do a few simulations, and viola, you can get a precise number of how you're likely to do. Plus handy tool to evaluate schedule quality.
So maybe include some slack.
Still, maybe you want to know how often will your schedule run long. So do some measurements of historical data to find out how your task estimates work out in practice.
Plug those in and do a few simulations, and viola, you can get a precise number of how you're likely to do. Plus handy tool to evaluate schedule quality.