Can someone plz explain this question from Oliver Lehman test

Submitted by raza82 on Tue, 01/29/2013 - 15:42

 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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sspawar

Wed, 01/30/2013 - 00:31

in 1st case uncertainty =  p-o = 18 , hence p-o/6 = 3 = SD, in 2nd case  uncertainty is 24 means SD = 24/6 = 4.


now variance = 1st activity = 9 and 2nd activity = 16


hence combined  V = 9+16=25.


and combined  SD = 5,


Thus combined / entire uncertainty path will be +/- 3 sigma = 2*3*5 = 30 days (option B)


 

raza82

Wed, 01/30/2013 - 05:28

Pawar, can you please clarify the last line in your answer, I am unable to get it.

 

Secondly what does the question exactly mean by duration uncertainity, can you please elaborate on that? I didnt understand the question. 

daso

Wed, 02/06/2013 - 17:45

"Thus combined / entire uncertainty path will be +/- 3 sigma = 2*3*5 = 30 days (option B)"

 

and 2 means because it is 2 activities?

sunitaPMP

Wed, 02/06/2013 - 20:41

so if it would have 3 activities then its 3*3*5=45?

steelproject2012

Fri, 02/15/2013 - 17:39

1st: get the SD (P-O/6) for the entire path:

2nd: uncertainty (as defined by the question) = P - O = 6*SD

Got it; thanks

peeyushm

Wed, 01/30/2013 - 06:08

 SD of entire path is 5 as per Pawar's comment.

(P-O)/6 =5

uncertainty is (P-O) = 30 

sgadewar

Fri, 02/15/2013 - 22:07

Let it make little simple for you, if the question says that if duration uncertainty is 18 then it is nothing but standard deviation. So let us calcualte the variance, which is nothing but square of deviation. In this case it will be square of (18) =  324. On similar lines find for 24 , what is the variance which is nothing but 576.

As you also know the std deviation for entire path is SQRT OF (sum of all variance ) so subsitiuting the formula SQRT (324 + 576) = SQRT (900) = 30

 

Hope this helps

 

Sameer 

sspawar

Sat, 02/16/2013 - 02:52

 Comment of Mr Peeyush is right :

 

SD of entire path is 5 as per Pawar's comment.

(P-O)/6 =5

uncertainty is (P-O) = 30 (it is the easy concept)

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Meaning of 2*3*5 = 30, is the same 

it means 

+3sigma = 3times of SD right hand side of mean of bell curve

-3sigma = 3times of SD left hand side of mean of bell curve

thus total length = total uncertainity P-O = (3SD of RHS) + (3SD of LHS) =( 3*5) +(3*5) = 2*3*5

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one email Q and  Reply

 

 Message:

 Hi pawar,

Can you pls explain your below answers?

 How did you arrive this variance of 9 & 16? Appreciate ur time.

 What is this combined SD?

How its 2*3*5?

 in 1st case uncertainty =  p-o = 18 , hence p-o/6 = 3 = SD, in 2nd case  

uncertainty is 24 means SD = 24/6 = 4.

 now variance = 1st activity = 9 and 2nd activity = 16

 hence combined  V = 9+16=25.

 and combined  SD = 5,

 Thus combined / entire uncertainty path will be +/- 3 sigma = 2*3*5 = 30 days (option B)

--------------------------------------------------------------

My Answer:

 Its simple u know

V =square of SD
When we approach for combined SD
It is procedure
1st we find out individual activity wise SD then convert them in Variance and then add those variances and then find out suare root of that added value of variances will be combined SD of those set of activities.
In this example u see
Sd1 = 4 V1 = 16
Sd2 = 3 V2 = 9
Sum of Vs = 25
SD of both activities combined = sauare root of 25 = 5.
Now question is asking about range quantum (p-o) for ±3sigma = ±3SD = means 3 times of sd left side and 3 times of sd rifgt side .  And thus its range quantum will be = 2*3*5= 30. Read question again and again .

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Nishu

Sat, 05/25/2013 - 12:39

 HI

I computed as foll

first dur uncertainity is 3sigma = 18

so sigma = 6

 

second 3sigma = 24

so sigma = 8

proj sd = sqrt( sqr(6) + sqr(8) )

= 10

 

duration uncertainity is 3sigma hence 3*10 = 30.

Pls convince me if this is wrong.