Previously I discussed the difference between gross velocity and net velocity and now I’d like to show why they’re important. A ranged burndown chart, an extension to normal burndown charts which apply both the gross and net velocity, is shown below. Where a burndown chart uses the (gross) velocity to predict a potential end date, and by extension gives a feel for the potential project cost, a ranged burndown gives a potential range of end dates/costs. Giving a ranged estimate is a known best practice in the IT community.
Because it’s possible that functionality can be dropped from a release part way through a project, perhaps because of a major shift in strategy or in an effort to hit a desired date, the net velocity will exceed the gross velocity that iteration. In this case our advice is the use the change in requirements from the previous iteration to calculate the net velocity.
Note that this blog posting is excerpted from Chapter 10 of the book Disciplined Agile Delivery.