Three Point Estimation or PERT Estimation Technique
Three-Point estimation is one of the techniques that could be used to estimate a task. The simplicity of the Three-point estimation makes it a very useful tool for a Project Manager that who wants to estimate.
This is a statistical method, but it also breaks down the task into subtasks (in this it is similar to WBS). Then, three possible scenarios should be estimated for each sub-task.
The best case: assuming that you have a talented team and all of the necessary resources, and assuming that no problem occurs and everything goes right, you can complete the task, for example, in 100 man-hours (B). This is an optimistic scenario.
The most likely case: assuming that you have a good team, enough resources, and almost everything goes right, although some problems may occur, you can complete the task in 150 man-hours (M). This is a normal scenario.
The worst case: assuming that your team is not experienced, everything goes wrong and you have to solve numerous problems, you can complete the task in 200 man-hours (W). This is a pessimistic scenario.
Thus, you have three values: B = 100 M = 150 W = 200
Now, you can calculate the average value for the test estimation (E) using the following formula:
E = (B + 4M + W) / 6
E = (100 + 4 x 150 + 200) / 6 = 150 man-hours
As the average value may fluctuate a little bit, to be more accurate, you need to calculate standard deviation (SD) — the limits within which E may change. The formula is as follows:
SD = (W – B) / 6
SD = (200 – 100) / 6 = 16.7 man-hours
You can present the final estimate as this: the team needs 150 +/- 16.7 person-hours to accomplish the sub-task.
Three-Point Estimation is one of the most effective methods for software testing when you have practice and data from the previous projects and an ability to apply them. The essence of this method is to find out the best and the worst working conditions for your team.