PERT is a time-honored tool in the project management field that usually is ignored by some of the simpler PM tools. PERT is, in essence, a fancy Critical Path Method (CPM) in which the times for the activities are given as Expected Times. In the classical approach, the expected times were computed by a weighted average:
Time(opt) + 4*Time(likely) + Time(pess) Time(expected) = --------------------------------------- 6
where Time(opt) is the optimistic time, Time(pess) is the pessimistic time and "6" is just the sum of 1+4+1. This assumes that the most likely time will be achieved in two out of three cases. For sophisticated project planners, the better way to weight the likely time for an activity is to use a statistical variance and standard deviations. This won't make the PERT chart any more believable, but it will be more defensible. Some high-end PM programs will let you incorporate these computations into a CPM network with relative ease. Of course with the HP Palmtops, statistical computations of this sort can be handled very easily in HP Calc.
Click on estimating times to read more about this topic.