On the sample questions in the PM Prepcast regarding PERT that I pasted below, why is it that the formula used to answer the question is the Beta distribution E=(O+4M+P)/6 instead of Triangular Distribution E=(O+M+P)/3? I answered letter C using triangular which is wrong as per below. I am now confused on when to use BETA and triangular formula. Please advise. Thanks
Question 8: A project requires securing government approval before the project work can begin. According to government policy it should take 30 days for the government to issue the approval provided that all necessary paperwork is complete. In the past, it has been observed that 45 days is the most likely turnaround time for such applications approval. If any objection is imposed due to incomplete paperwork, it may take up to 90 days to secure the approval. What would be the expected activity duration using PERT?
A.) 30 days
B.) 45 days
C.) 55 days
D.) 50 days Correct answer is D
Explanation: PERT is a Three-Point Estimate. The PERT formula is "Duration = (Optimistic + 4*Most Likely + Pessimistic)/6". Using this formula, we get an expected duration of 50 days.
Reference: PMBOK Guide 5th Edition, page 171
Revisiting this thread (there are only 3 entries on this topic).
I ran into a similar question about when to use Triangular vs. Beta Distribution method on the Formula Study Guide (# 58 in the 105 Questions). I don't specifically see guidance to use one formula over the other in the PMBOK guide page 170 or 171. However, I do see that the Beta Distribution has, in parentheses, "from the traditional PERT technique" next to the formula label.
i.e., Beta Distribution (from the traditional PERT technique). tE = (tO + 4tM + tP) / 6
Am I to read this as -- use the Beta Distribution formula unless specifically instructed to use the triangular distribution?
Ryan, in my experience, I have found PMP study prep and simulation questions used the PERT rather than the triangular. I honestly do not recall getting a question on a simulation or on my exam requiring the triangular formula. If others have experienced similar situations, then there should be no reason why you should see something different. Now don't forget any question about the PERT will most likely have some key words such as "optimistic" so you'll know to use that formula.
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