Does evidence based scheduling work right with heterogenous estimations?

Observing one year of estimations during a project I found out some strange things that make me wonder if evidence based scheduling would work right here?

• individual programmers seem to have favorite numbers (e.g. 2,4,8,16,30 hours)
• the big tasks seem to be underestimated by a fix value (about 2) but the standard deviation is low here
• the small tasks (1 or 2 hours) are absolutely wide distributed. In average they have the same average underestimation factor of 2, but the standard deviation is high:
  • some 5 minute spelling issues are estimated with 1 hour
  • other bugfixes are estimated with 1 hour too, but take a day

So, is it really a good idea to let the programmers break down the 30 hours task down to 4 or 2 hours steps during estimations? Won't this raise the standard deviation? (Ok, let them break it down - but perhaps after the estimations?!)

• Yes, your observations are exatly the sort of problems EBS is designed to solve.
• Yes, it's important to break bigger tasks down. Shoot for 1-2 day tasks, more or less.
  • If you have things estimated at under 2 hrs, see if it makes sense to group them. (It might not -- that's ok!)
  • If you have tasks that are estimated at 3+ days, see if there might be a way to break them up into pieces. There should be. If the estimator says there is not, make them defend that assertion. If it turns out that the task really just takes 3 days, fine, but the more of these you have, the more you should be looking hard in the mirror and seeing if folks aren't gaming the system.
  • Count 4 & 5 day estimates as 2x and 4x as bad as 3 day ones. Anyone who says something is going to take longer than 5 days and it can't be broken down, tell them you want them to spend 4 hrs thinking about the problem, and how it can be broken down. Remember, that's a task, btw.
• As you and your team practice this, you will get better at estimating.
• ...You will also start to recognize patterns of failure, and solutions will present themselves.
• The point of Evidence based scheduling is to use Evidence as the basis for your schedule, not a collection of wild-assed guesses. It's A Good Thing...!