A few days ago I wrote about ranged burndown charts. Interestingly, if you track the ranges over time you end up with a chart such as the one below which corresponds to the estimating cone of uncertainty (depicted by the dashed lines). It’s interesting to note that this example includes two common occurrences that you’ll see. First, during iterations one and two the gross and net velocities were the same because no new functionality had been identified yet, resulting in an unranged estimate. Second, iteration eight had a very small net velocity because the amount of new functionality was almost as much as the amount implemented, giving a huge estimation range due to the small net velocity.
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.
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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, the topic of my next blog posting.