Estimating a task and estimating a program of work are completely different things. Task estimation is only ever accurate in aggregate. If you can estimate free

Estimating a task and estimating a program of work are completely different things. Task estimation is only ever accurate in aggregate. If you can estimate free of outside influence (ie gaming the system) then you can assume an equal amount of over and underestimation making week by week velocity relatively stable. Outliers happen no matter what. Estimating a program is mostly futile. Not because task estimation is impossible but because project requirements are moving target. That's pretty much the whole point of agile. Ability to adjust to changing priorities comes at the cost of predictability but it's a worthy tradeoff because building the right thing trumps everything else. You can measure your completed backlog against the outstanding backlog to get an idea of progress and forecast when you'll hit the 37th next priority but that isn't necessarily going to the day that you're "done" because priorities shift.