If you think the odds are good that a task will come out significantly ahead or behind of schedule, you may want to adjust PERT weighting. This sets the odds of each duration occurring. By default, Project gives the pessimistic and optimistic scenarios each a weight of one, while the expected duration is weighted at 4. In other words, the expected duration will be fulfilled four out of six times, while the optimistic or pessimistic scenario will occur only one out of six times.

 

To change the weighting, select one or more tasks in any view and click the PERT Weights button () on the PERT toolbar.

 

 

Now, you can weight each scenario as you would like. Just remember that the total of the three numbers must be six.