The Special Projects Office of the US Department of Defense developed the Program Evaluation Review Technique (PERT) as part of the Polaris submarine-launched ballistic missile programme. PERT made use of the recently developed Critical Path Analysis techniques, and supported calculations of the time needed to complete projects through a systematic analysis of the tasks involved.
PERT attempts to address timescale uncertainty in projects by using three different probabilistic time estimates for each project component: optimistic time, most likely time and pessimistic time for completion. Calculations are then performed to arrive at the PERT weighted average time for each task, giving more weighting to the most likely time than the other two - the actual formula is:
(optimistic time + (4 x most likely time) + pessimistic time)/6.