Data Type Date
Entry Type Calculated or entered
Description The Pessimistic Start field contains the date that represents the worst-case possibility for when a task could start. You can enter this date as part of a PERT analysis calculation, which takes a weighted average of expected, pessimistic, and optimistic dates and durations. This field is available only in the PA_Pessimistic Case table or the PA_Pessimistic Gantt view.
How Calculated When you first create a task, the Pessimistic Start date field contains "NA." When you enter a pessimistic duration and pessimistic finish date, and then click Calculate PERT 
Best Uses Use the Pessimistic Start field in the PA_Pessimistic Case table or the PA_Pessimistic Gantt view to estimate the probabilities of task dates. By default, the PERT analysis calculation gives heaviest weight (4) to the expected information, and the lightest weight (1 each) to the pessimistic and optimistic information.
Example You have several critical tasks for which you want to see the best-case, worst-case, and expected start dates. You enter the expected finish dates and durations for these tasks, as well as their optimistic and pessimistic finish dates and durations, and then run the PERT analysis calculation.
Remarks By changing the weight that Microsoft Office Project attributes to each of the three estimated durations, you can tailor the probable estimate to be more accurate. To change the weight, click PERT Weights 
The Pessimistic Start field uses the Start3 custom start date field.
