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Software Development and Project Management

A three-point estimate of an activity's duration gives an optimistic value of 4 days, a most likely value of 7 days and a pessimistic value of 16 days. What is the expected duration given by the PERT formula?

  1. A7 days
  2. B8 days
  3. C9 days
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Answer: B. 8 days

The PERT three-point estimate gives the expected value as (optimistic + 4 × most likely + pessimistic) ÷ 6, so (4 + 4 × 7 + 16) ÷ 6 = 48 ÷ 6 = 8 days. It is longer than the most likely value of 7 days because the pessimistic value lies far away. The standard deviation, an indicator of the spread, is (pessimistic − optimistic) ÷ 6 = 2 days. 9 days is the simple average of the three values, (4 + 7 + 16) ÷ 3.

  • A. This is the most likely value itself; it does not include the effect of the pessimistic value lying far away.
  • B. Correct. (4 + 4 × 7 + 16) ÷ 6 = 8 days.
  • C. This is the simple average of the three values; PERT gives the most likely value four times the weight.
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