Why PERT is known as probabilistic in nature? Explain the different PERT time estimates.

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written 8.3 years ago by

teamques10 ★ 68k

In CPM, a fixed duration is used for each activity based on previous experience and knowledge. In that sense, it can be deterministic.
Also, in CPM, the critical path is determined on the basis of critical activities i.e. float calculations are performed for all activities to find out the critical activities, and the critical path. Hence CPM is an activity oriented network.
On the other hand, in PERT, three probable time estimates based on probability distribution are assigned to an activity, which then give the most likely time estimate. Hence, PERT can be said to be probabilistic network.
Also, in PERT, the critical path is determined on the basis of critical events i.e. slack calculations are done to determine path is determined on the basis of critical events i.e. slack calculations are done to determine the critical events and hence, the critical path.
Hence PERT is an event oriented network.
The different PERT time estimates are as follows:-
- Optimistic Time (to):- The Optimistic Time is an estimate of the minimum time required for an activity if everything goes as per the ideal condition i.e. if better than normal conditions prevail.
- Pessimistic time (tp):- The pessimistic time is an estimate of the maximum time required if unusually bad luck is experienced. It may take account of an initial failure or delay but should not be influenced by major hazards unless these are inherent in the activity.
- Most likely time (tm):- The most likely time is based on experience and judgment, being the time required if normal conditions prevail. It lies in between optimistic time and pessimistic time.
- Expected mean time (te):- $$t_e = t_o + t_m + t_p/4$$

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