Including Uncertainty in Estimates of Software Projects, Fort Building, and anything including a Toddler

Early in a project, so many of the specific details of the nature of the software being built, specific requirements, project plan and staffing details are all very unclear. Because there are so many variables early on in the project, it is crucial to include a large degree of uncertainty or variability in the project estimate. This is not about being purposely misleading or avoiding commitment to an exact number with your stakeholders, this is about accepting the reality of software projects that leave so much to be defined early on. To commit to an exact number at the very beginning would be misleading yourself and your stakeholders and presenting a false sense of confidence in something that still has so much yet to be defined.

Steve McConnell, CEO and Chief Software Engineer at Construx Software, presents the idea of a “Cone of Uncertainty” in his book “Software Estimation: Demystifying the Black Art (2006)”.

The horizontal axis shows significant project milestones. The vertical axis shows the degree of error that has been found in estimates created by skilled estimators at various points in the project. What is obvious from the diagram is that estimates created early on in the project are subject to a high degree of error (from .25x lower to 4x higher). As the details of the project become defined and understood, the cone narrows. Obviously the most accurate estimate is made at the very end of the project development, but the challenge in the software world is to find somewhere in between where we know enough about the project to make the best estimate possible while still allowing major stakeholders to plan financially. More about the Cone of Uncertainty, and other estimation resources can be found on the Construx website.

In his book Steve McConnell explains several different techniques used in making software estimates. He also made a very interesting and entertaining blog post recently, where he shared similarities between building a fort in his backyard and problems people run into with software estimates. The general idea here, and very humorously explained, is that in the beginning of a project it’s easy for us to assume that everything will go as planned and the project will proceed smoothly and in a timely manner, but it’s very common for things to take longer than expected. In his case, it was the little construction project in his backyard.

I haven’t built any forts lately, but I’ve managed many projects, and some that have taken longer than the original estimate, for one reason or another. But I also see this concept clearly illustrated in my day to day life outside of my work. I find that it’s almost impossible to make any type of time or schedule commitment when a toddler is involved. I’m fortunate to be the mother (or project manager) of two little girls, and have the pleasure of bringing the older one to preschool every morning. What should take only 15 minutes, can sometimes take up to 40…and this is why:

1. The 2nd and 3rd bowl of cereal (6 mins)

2. Trip to the potty before leaving home, which can sometimes include the mandatory reading of the Dr. Seuss book while waiting for the potty business to complete. (8 mins)

3. Sneakers that get taken off and put back on again, only to get taken off one more time (and of course put back on again) before the final trip out the door. (2 mins)

4. Unexpected meltdown about which jacket to wear, and wanting to wear rain boots and bring umbrella on a perfectly sunny day. (6 mins)

You catch my drift…

So, I’m learning more about how to properly include uncertainty in my estimates at the appropriate times in the project development, both in the projects I manage and the mini-projects I manage at home every morning. It’s a good thing that our preschool allows us a 30 minute window for morning drop-off!

(This blog post was originally published in May 2008 on Flightpath's Digital Insight Blog)