If you say something will take an hour, you're almost guaranteed to be wrong, at best you might fall somewhere near that, but you're almost certainly not going

If you say something will take an hour, you're almost guaranteed to be wrong, at best you might fall somewhere near that, but you're almost certainly not going to take exactly an hour. I think that estimating in best case / worst case ranges is useful for two reasons. First it lets you convey uncertainty. Instead of just double a number you made up, you can let people know where you're very certain (narrow range) and totally guessing (wide range). The second valuable thing about ranges is that you can use them to make estimate about projects as a whole. If a project contains a lot of uncertainty, it's results are likely to be uncertain. We use this technique at LiquidPlanner to generate more realistic schedules, which is pretty damn cool. The second best thing to do when estimating is update your estimates whenever they change. If you learn something that is going to affect a ship date, speak up!