18 + 4x + 24

__________    =  x

            6

= 21

I am lost in question. Please explain the question and than answer.

Here P-O is 18 and 24. So standard deviation is P-O/6 i.e. 3 and 4. Sigma level is 3. so Activity standard deviation is 9 and 12. Activity variance is 81 and 144. Total is 225. Standard deviation for the project is  sq root of 225 which is 15.

I think I am somewhere wrong. Please explain.

Definition (Rita):

Uncertainity is the lack of knowledge about an event that reduces confidence in conclusions drwan from the data.

Note, 3 Sigma = 99.73%, seems to be a distracter(as Rita puts it). I can think of two solutions, but not sure what exactly is it.

SOLUTION 1:

The duration of uncertainity (Pessimistic-Optimistic) is already calculated for the activities, so just adding up the two durations would give total duration of uncertainity for entire path.

Duration uncertainity of entire path = duration uncertainity of activity 1  + duration uncertainity of activity 2

Duration = 18+24 = 42

ANSWER: D

SOLUTION 2:

STEPS:

1. Calculate standard deviation of activities; SD = (P - O) / 6

2. Calculate variance of activities; variance = SD²

3. Calculate Total variance

4. Calculate Uncertainity; U = P-O

Given, uncertainity => U = P-O

Given, standard deviation => SD = (P - O) / 6

Given, variance = SD²

Given A1 = 18, A2=24

STEP 1:

SD = U/6

SD1 = A1/6 = 18/6 = 3

SD2 = A2/6 = 24/6 = 4

STEP 2:

V1 = SD1² = (3)² = 9

V2 = SD2² = (4)² = 16

STEP 3:

Total V  = V1 + V2 = 9 + 16 = 25

STEP 4:

Given V = SD², or

25=SD² => SD = 5

U = (P-O)

SD = U/6, or

U = SD x 6 = 5 x 6 = 30

ANSWER: B

 Need someone whos an authority on PMBOK, to "authenticate" the correct answer.

Does this sort of question "REALLY" a part of PMP exam?

 Hi,

Before I go to the answer I would like to share my view on the question itself:

First of all, I would respectfully state that the question is not stated correctly. The terms 1sigma, 2sigma,…, 6sigma and their respective population percentages all applies to data that is normally distributed (in other words the sampling distribution and population distribution forms a normal distribution). Formulas for statistics such as standard deviation are different for different statistical distributions. To apply normal distributions we need at least 30 samples (if we are unsure regarding the population distribution). At lest 30 samples means at least 30 points.

When we don’t have at least 30 points, we can’t use normal distribution to estimate population parameters. PERT is based on 3-point estimate. Since we can’t use normal distribution for PERT, we use triangular distribution as this distribution is based on 3-points (a triangle).

I would only accept this question, if the term 3sigma was used to create a distraction. Even if that’s the case, you won’t find similar clashing-concepts distractions in the real PMP exam. If there should be any distraction, it should be totally irrelevant to the question. Relax you won’t find such a question on the exam.

Okay let’s assume that the term 3sigma was a distracter; let’s focus on the remaining part of the question. It is stated that the network has only two activities. First activity’s uncertainty is 18 days. Then it is mentioned that the “the second estimate has an uncertainty of 24 days”. Now this term is ambiguous. Does the author want to state that the “second activity’s” uncertainty is 24 days or the secondary estimate of the first activity resulted in the uncertainty of 24 days? Since the first one make more sense, I would again assume that it’s the uncertainty of the second activity.

We can’t add the uncertainties together to get the project uncertainty. I would agree with the “standard deviation” technique used by a couple of people over here, yielding a total uncertainty of 30 days. But for that I again have to make an assumption. If I assume that these two activities have a finish-to-start relationship, I would say, based on the “standard deviation technique”, the total uncertainty is 30 days. But what if the two activities have a start-to-start relationship? Nobody thought of this? In that case the total uncertainty will be the greater of the two uncertainties, i.e., 24 days.

In short, I would have to make a number of assumptions in order to say that the total uncertainty of the project is 30 days. Hence, I would go for the last choice, that is, “No statement is possible from the information given”. I would go for this choice since this one makes more sense to me.

Regards,

Exam Support Team

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