Wrong answer..please do advice
The following three tasks form the entire critical path of the project network. The three estimates of each of these tasks are tabulated below. How long would the project take to complete expressed with an accuracy of one standard deviation?
| Task | Optimistic | Most likely | Pessimistic |
| A | 15 | 25 | 47 |
| B | 12 | 22 | 35 |
| C | 16 | 27 | 32 |
- 75.5
- 75.5 +/- 5.08
- 75.5 +/- 8.5
- 75.5 +/- 2.83
Answer: B
Explanation: The critical path is the longest duration path through a network and determines the shortest time to complete the project. The PERT estimates of the tasks listed are 27, 22.5 & 26. Therefore, the length of the critical path of the project is 27+22.5+26 = 75.5.
The standard deviations of the estimates of the three tasks are determined as (P-O)/6 = 2, 3.83 & 2.67 respectively. The standard deviation of the total path is determined as Sq. root (sum of variances), where variance = square of standard deviation. Thus, standard deviation of critical path = Sq. root (4+14.67+7.13) = 5.08.
The Answer is wrong. The Standard deviation should be P-O but for task A the author has calculated P-M/6 ...please do advise
PMPBOY
Sat, 05/21/2011 - 19:19
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Yes answer is wrong for s.d.
Based on the data given in question, s.d. is wrongly calculated.
The correct s.d. will be as shown below:
The standard deviations of the estimates of the three tasks are determined as
(P-O)/6 = 5.33, 3.83 & 2.67 respectively, and not (P-O)/6 = 2, 3.83 & 2.67 respectively.
The standard deviation of the total path is determined as Sq. root (sum of variances), where variance = square of standard deviation.
Thus, standard deviation of critical path = Sq. root (28.41+14.67+7.13) = 7.09.
So correct answer will be 75.5 +/- 7.09.