Three point estimation

As it happens, I often have to do estimates for small to kind of large projects. As most developers will know, estimating is not as easy as you think. It is even harder to guess-timate something you have never done before. During my project management classes, I did learn about three point estimation, a pretty useful tool which can help anyone get a more balanced estimate of how long the task or activity is likely to take.

Gantt Chart

As for all estimations, we need to first break down the project into individual tasks or activities. Once that is done, we need to decide on the time it will take to complete each task. However, rather than using a ballpark estimate, we can use three point estimating to get a more accurate idea. To do a three point estimation for a particular task or activity we need to get three different numbers: the most optimistic estimate, the most pessimistic estimate and the most likely estimate. Once we have these numbers, its possible to calculate the real value based on the three point formula. Start by adding your optimistic time to your your pessismistic time and then add four times the most likely time. Take the total and divide it by six. This will give us a result which is more likely to tell us exactly how long completing the task or activity is going to take.

The three point estimation formula

E = (O + P + 4 * L) / 6
O = most optimistic estimate
P = most pessimistic estimate
L = most likely estimate

For example, to write a blog post I could say that

O = 10 minutes
P = 60 minutes
L = 20 minutes

Thus my estimate is: (10+60+4*20)/6 = 25 minutes