InfoQ offers an interesting read on agile planning and estimation. Here are a few key quotes
All numbers are based on assumptions. Your figures are only as good as your estimates and guesses – and there may be serious flaws in your assumptions.
If we approve a project based on a year’s worth of work, and we estimate it and we think it is all good, there is a lot of risk inherent in there. But also, and this is worse, we might ditch doing something really, really valuable for the company because we have clumped together all this value into a year-long project and we are making a really big decision – to go or no go – based only on that clump.