PERT Estimate = (P + 4(R) + O) / 6
That is to say, take your pessimistic estimate (worst case estimate if everything goes totally wrong), plus 4 times your realistic estimate (what you think it will most likely take), plus your optimistic estimate (if everything goes perfect) and divide that sum by 6.
Now, if we throw probability into the mix, recognize that you only have a 50% chance of completing the task within the time frame that the PERT estimation formula provides.
To increase the probability, find the standard deviation by utilizing this formula:
Standard Deviation = (P - O) / 6
That is to say, take your pessimistic estimate and subtract your optimistic estimate and divide the result by 6.
Since the PERT estimate result yields a 50% probability, an 84% probability can be achieved by adding one standard deviation to the PERT estimate. A 97.5% probability can be achieved by adding two standard deviations to the PERT estimate, and a 99.5% probability can be achieved by adding three standard deviations to the PERT estimate.
For example, let's assume that you are driving from point A to point B. Your pessimistic estimate is 8 hours, your realistic estimate is 4 hours and your optimistic estimate is 2 hours. First complete the PERT estimate:
(8 + 4(4) + 2) / 6 = 4.333
Now calculate the standard deviation:
(8 - 2) / 6 = 1
The result:
You have a 50% probability of completing the trip in 4.333 hours
You have an 84% chance of completing the trip in 5.333 hours
You have a 97.5% chance of completing the trip in 6.333 hours
You have a 99.5% chance of completing the trip in 7.333 hours
And this is how PERT estimating, aligned with Standard Deviation, can provide you with an estimating methodology that you can use in almost any type of planning exercise.
No comments:
Post a Comment