## duration uncertainty

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wahiba
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Joined: Tue Jan 28, 2014 1:59 pm

### duration uncertainty

Oliver F. Lehmann 175 free test

question 94 says:

Together with your team, you applied 3 points estimation on a critical path which consists of two activities.

The following duration uncertainties are all calculated assuming a +- 3 Sigma confidence interval.

The duration uncertainty_ define d as pessimistic minus optimistic estimate_ of the first activity is 18 days; tge second has an uncertainty of 24 days.

Applying the pert formula for paths, what is the duration of tge entire path?
saket
Posts: 3113
Joined: Sat Dec 07, 2013 10:37 pm

### Re: duration uncertainty

Be careful with this question set the real exam is not as hard as this...

1. In order to solve this question you need to remember that formula for standard deviation (sigma ) is (Pessimistic - Optimistic )/ 6

2. When you have info that this is done at sigma level or % confidence level use sigma value to calculate the pessimistic and optimistic duration.

3. When you have multiple activities in your critical path and you need to calculate the sigma at overall level (sum level) do not sum the sigma of all activities rather sum the variance (sigma *sigma) of all activities and then calculate the sigma at path level.

4. Since only two activities and no info about the sequence is given you need to assume they are serial

5. Duration Uncertainty is defined here as Pessimistic - Optimistic which is equal to 6*sigma , so you need to calculate the sigma of these two activities combined and multiply it by 6

Question94-Sigma.gif (21.53 KiB) Viewed 5419 times

Hope this explanation will help.
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namita
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Joined: Wed Dec 11, 2013 10:25 am

### Re: duration uncertainty

Nice Explanation Saket
wahiba
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### Re: duration uncertainty

But what is the variance (sigma *sigma)
namita
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Joined: Wed Dec 11, 2013 10:25 am

### Re: duration uncertainty

Variance is always Sigma Square
saket
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Joined: Sat Dec 07, 2013 10:37 pm

### Re: duration uncertainty

Variance is Square of Sigma = ((P-0)/6)^2
Variance = (Sigma)^2
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wahiba
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Joined: Tue Jan 28, 2014 1:59 pm

### Re: duration uncertainty

Ok I got it
wahiba
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Joined: Tue Jan 28, 2014 1:59 pm

### Re: duration uncertainty

Always or when there is more than one activity?
saket
Posts: 3113
Joined: Sat Dec 07, 2013 10:37 pm

### Re: duration uncertainty

It is always, when we have more than one activity and we want to know variance at path level... we need to sum the variance and in order to calculate the sigma we need to take square root of variance.
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