Today I heard interesting thing. Our deadline for committing to plan for next year was moved to later, some time in the middle of June instead of being as planned. "I hope it will help us to get better estimates" we were told (this was my interpretation, the original was probably a bit different).

I was quiet as I would not be able to change that decision in any way. Still I wonder how giving us 2 or 3 weeks more will improve our ability to estimate 12 following months. Why management feels better to guess what 20 people can do in 12 months than estimating only 1-2 months, performing planned tasks and then checking where we got.

But that is not the reason for this post. It reminded me important information I have found in Implementing Lean Software Development: From Concept to Cash some time ago.

Goldratt believes that the key constraint of projects - he considers the product development to be a project - is created when estimates are regarded as commitments. [...] Since the estimate will be regarded as a commitment, the estimator accommodates by including a large amount of a "safety" in case things go wrong. However, even if things go well, the estimated time will be used up anyway, since estimators don't want to look like they over-estimated.

and bit later in the same chapter:

In fact, if half of the activities don't take longer than their estimated time, the system will not achieve the desired improvement.

This is that important bit of information - when all tasks are achieved in estimated time we cannot be sure we did not waste time. But when some of them (ideally 50%) take longer than estimated time then one of the possible reasons could be that our estimates are very close to minimal time needed, thus waste is minimized.