A few years ago, in Dr. Dobb’s Journal I wrote about estimating on agile development projects. In that article I discussed burndown charts and how to extend them to show an estimation range. The basic observation is that there is really two velocities exhibited by a team, the gross velocity and the net velocity. The gross velocity which is the amount of work they complete in an iteration, which is what a regular burndown chart shows. The net velocity is the change in the amount of work still to do, which is the amount of work completed in an iteration less the added amount of functionality that iteration.
So, as the diagram depicts if a team completes 20 points of work in an iteration but 5 extra points of work was added by the stakeholders, the gross velocity is 20 points whereas the net velocity is 15 points. If there’s 230 points on the stack then the gross velocity implies that there are 12 iterations left and the net velocity 16 iterations, providing you with a ranged estimate.
Given that we now have two velocities to chart, not just one, this leads us to evolve burndown charts into what is called ranged burndown charts.