Tasking developers with creating detailed estimates is a waste of time (2020)
An estimate is usually the sum of a bunch of smaller estimates. Those estimates have a probability distribution that is asymmetric. While sometimes they can be quicker than anticipated, the lowest they can go is zero (turns out we don't need it, cut it) and that kind of thing is relatively less common than a blowout the other way. Those blowouts are uncapped, while zero time is a minimum there's no maximum. The unknown unknowns are the things that get you. Seems like it will take X but the frobnicator was broken in ways we couldn't have anticipated so we had to get a different thing and then hack it to make it do what we actually needed etc etc. Each individual item has a low probability of that happening to some extent. When you have a bunch of things in sequence one of them probably will if not more.IIRC each item is described by a poisson distribution. (Have I got that right, mean, long tail). Nobody models estimates as a sequence of dependent, poisson distributed events, that I've heard of at least. If you did you might at least get a range on the estimate. Between 2 and 20 days. That would be more accurate. Useful to anyone much? Unclear.