Published on March 7, 2012 by Joel Milne
Enter the PERT estimation formula. Originally used in the software industry, it is also well-suited to website development and creative work estimations. The formula itself is very simple to learn and use:
Let's look at the PERT formula again in its standard short written form:
PERT estimating example
Let's say your task is to write a blog post such as the one I'm writing right now. So let's say in the past I've been able to usually write an entire post from start to finish in around 20-30 minutes. So I estimate my most likely time to write a blog post is around 25 minutes. Next I imagine the O (optimistic) estimate. If I write a short post, and the research goes very well, and ideas flow well, I might complete a post in as little as 12 minutes. That is the absolute best-case scenario I could imagine for the task. So I enter 12 as my O value in the PERT formula.
Finally we estimate the worst-case scenario, the P (pessimistic) view. If I have difficulty researching my topic, get hit with some form of writer's block, and find drafting my blog post difficult, I might end up using as much as 50 minutes on a single post. So my pessimistic estimate is double my most likely scenario. Now it's important to point out this is a task I have done before, and its one with a fairly routine and simple process. If this was a task I had never done, or it involved several unknown factors, my "safe" high estimate might be anywhere from 3-10 times my most likely estimate.
Let's put it the estimated numbers together into the PERT formula:
(12 + (4 X 25) + 50) / 6 = 27
The result is an estimate of 27 minutes to complete the task of writing 1 blog post. This ends up being quite close to the most likely (ML) of 25 minutes. Yet while adding a mere 2 minutes to the task estimate may seem almost trivial, consider that in the long-term that actually represents a buffer of around 7% compared to simply estimating the expected time to complete. On tasks where your pessimistic estimate is high because of many potential pitfalls, you might find that your buffer percentage is significantly higher.
I hope you have enjoyed learning about the PERT formula and find it useful in your work. Now when people ask how you have reached your estimation for a project instead of telling them "I just guessed" you can tell them "I guessed 3 times and used a proven scientific formula to balance and average the numbers". Yay PERT!