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In project scheduling probability considerations are incorporated by assuming that the time estimate for each activity is based on 3 values. The three values are, the optimistic, pessimistic and the most likely time.
The optimistic time will be required if the execution of the project goes extremely well. The pessimistic time will be required if everything goes bad and the most likely time will be required if the execution is normal.
The probability distribution of times for completing an event can be approximated by the normal distribution due to central limit theorem. Probability can be seen as the project reliability. Given a certain project cost, the probability of success of a project can be determined by putting into consideration the resource availability.