TOPIC: Calculating Critical Path variance

I would be sincerely grateful if you could answer the following question. After calculating task duration with a PERT three point estimate do you include lag on the total task duration to calculate variance or just the PERT estimate without the lag time? Hope this is clear. Many thanks for your help in this matter.
Regards
Paul Evans

Hi Paul Evans.
To calculate task duration using the Project Evaluation Review Technique (PERT), three-point estimate you need to have 3 parameters which are:-
1) Optimistic duration estimate = a
2) Pessimistic duration estimate = b
3) Realistic duration estimate = m
To get the task duration, you give a weight of 4 to the realistic time duration (this means to multiply it by 4) and add the other 2 parameters and then divide the answer by six.

Therefore:- Estimated Time (T) = (a + b + m)/6

Variance is then given by the formula:-
Variance x Variance = (a-b)(a-b) x 1/6

(Please note that due to my inability to paste fractions and superscript to my answer above, I have made the formula for variance to be a bit more complex than it really is so that it can remain mathematically correct. The right side denotes the square of variance and the left side denotes be the square of the difference of the Pessimistic and Optimistic time divided by 6)

As you can see from the formulas above, the lag time does not feature in the workings and to answer your question; lag on the total task duration is not used to calculate variance. I hope that I have answered your question.

Many thanks for your prompt reply and clear explanation.