PERT Estimation Technique

PERT (Program Evaluation Review Technique) is an estimation technique that was first developed and applied by the United States Defence establishment for their Ballistic Missile development program. It was one of their most ambitious programs. Completion of this in time ahead of the other nations was critical for them. Such a missile development program was filled with a huge amount of uncertainty, as it required a large number of supplier agencies working on new technology development. This method of estimation helped them build in all the uncertainties in their estimates and helped them to complete this program ahead of their expected schedule.

PERT uses a three-point estimation approach for a task. Any task filled with uncertainties can have a wide range of estimates in which the task actually will get completed. Uncertainties include both favorable conditions (opportunities) as well as unfavorable conditions (threats). 

PERT includes statistical analysis.

The 3 points of the estimate are as below:

• Optimistic estimate – Estimate when all favorable things will happen (all opportunities happen and no threats take place)
• Pessimistic estimate – Estimate when all unfavorable conditions happen (all threats happen and no opportunities take place)
• Most Likely estimate – Estimate when both favorable and unfavorable conditions will happen

For example, if we think about a task that involves traveling in a crowded city from a specific location in the city to the airport. The actual time taken will depend upon the traffic condition on the road. 

This may take optimistically 30 minutes, pessimistically 90 minutes, and most likely 60 minutes. So the range in which the travel time will fall is 30 minutes to 90 minutes. 

An average expected estimate is calculated by taking a weighted average of these 3 points of estimates using the below formula:

• E (Mean PERT Average) = (O+4ML+P)/6   (by giving more weightage to the most likely estimate)
• Standard Deviation (SD) = (P-O)/6

The actual time taken to travel will be anything within this range with the most likely duration as 60 minutes. PERT allows applying statistical concepts to study the probabilities of completing the tasks in a particular estimate range.

 Assuming that the mean is exactly in the middle, and assuming the actual estimates will follow a normal distribution within the extreme range of optimistic and pessimistic estimates, we can predict the following probabilities of completing the task in a certain range of time as below:

• Mean +/- 1 Standard Deviation range – Probability is 68.4%
• Mean +/- 2 Standard Deviation range – Probability is 95.5%
• Mean +/- 3 Standard Deviation range – Probability is 99.7% 

Conclusion

PERT estimation technique will be a practical approach for estimating when the tasks on hand are filled with uncertainties, where the tasks may take up different estimates depending upon certain conditions. Project management professional certification is a perfect way to learn this technique while preparing. The actual estimate is dependent on certain variables. PERT allows the preparation of a more practical estimate by factoring the 3-point estimates into one as explained above.