The problems he talks about have noticeable effects on task completion time and estimate error. In my experience, estimation error is not normally distributed.

The problems he talks about have noticeable effects on task completion time and estimate error. In my experience, estimation error is not normally distributed. For most tasks, estimation error is small, but perhaps one in ten estimates are off by an order of magnitude. It's usually a single bug or feature which blows the iteration. Though, by consuming so much time, it may cause other tasks to spill too. If you aim to estimate most tasks accurately, the overall project will probably be underestimated. Outliers consume a significant portion of the overall project time.