PERT

[Meenakshi Naidu]

Can someone help me understand this question?

Together with your team, you applied three-point estimation on a
Critical path which consists of two activities. The following duration
uncertainties are all calculated assuming a ±3sigma Confidence
interval. The duration uncertainty—defined as pessimistic minus
optimistic estimate—of the first activity is 18 days; the second
estimate has an uncertainty of 24 days. Applying the PERT formula for
paths, what is the duration uncertainty of the entire path? 94
o 21 days
o 30 days
o 42 days
o No statement is possible from the information given.

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regards,
Meenakshi

[DHIRAJ KALRA]

Answer is 30 days

STEP 1:

SD = U/6

SD1 = A1/6 = 18/6 = 3

SD2 = A2/6 = 24/6 = 4

 

STEP 2:

V1 = (SD1)squared = (3)squared = 9

V2 = (SD2)squared = (4)squared = 16

STEP 3:

Total V  = V1 + V2 = 9 + 16 = 25

STEP 4:

Given V = (SD)squared, or

25=(SD)squared => SD = 5

U = (P-O)

SD = U/6, or

U = SD x 6 = 5 x 6 = 30

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Dhiraj Kalra

[Sibhi Srinivasan]

SD1=18/6=3
SD1=24/6=4
SD=7

+-1 Sigma means 1SD
+-2 Sigma means 2SD
+-3 Sigma means 3SD

So, 3*7 = 21

Thanks
Srini

[Raghunandan Kulkarni]

Answer is 30 days.

Please note that when calculating total standard deviation you should not directly add individual SD rather calculate variance and square root of variance will give you final final SD. 

SD = (P-O)/6 therefore P-O = 30 days.