The Program Evaluation and Review Technique (PERT for short) was developed by the United States military in the 1950s, it is sometimes referred to as a “Three Point Estimate.” It is a technique that provides a weighted average for estimating the duration of an activity. The logic behind the usage of this technique is to provide a higher level of accuracy for estimating activity duration, especially those with a high level of uncertainty.
The reason it is sometimes referred to as a “Three Point Estimate” is that three estimates for activity duration are taken into consideration. The first is the Optimistic estimate, which is the best case scenario for completing the activity; this is the value with the shortest duration. The second is the Pessimistic estimate, which is the worst case scenario for completing the activity (excluding catastrophic events like an earthquake or fire etc.) and is the value with the longest duration. The final value is the Most Likely estimate, which is the duration scenario most likely to occur. Once all the estimates are collected, the PERT formula is utilized to calculate the activity duration, it is as follows:
T = OE + 4ML + PE / 6
An activity identified on the project requires the PERT Technique to be utilized for estimating its duration. The following values have been identified.
Optimistic Estimate = 5 Hours
Pessimistic Estimate = 21 Hours
Most Likely Estimate = 9 Hours
Please use the PERT Technique to calculate the duration for the identified activity.
T = (5 + 4(9) + 21)/6
T = 62/6
T = 10.33
In the case above, the estimated duration of the activity using the PERT Technique is 10.33 Hours to completion. The primary benefit of this technique is that it uses a weighted average; it is biased towards the most likely estimate, as this value is always multiplied by 4. However, it also takes into account the best and worst case scenarios in terms of activity duration. The sum of these values are then divided by 6, which represents the total number of values collected; this gives you the PERT estimate. Overall, the Program Evaluation and Review Technique is very effective when estimating the expected duration of an activity (or set of activities) with a high degree of uncertainty.