Another Probability Simplified Solving Technique

Submitted by projmanpro on Mon, 04/23/2012 - 11:13

One asks me to offer a simplified probability calculation technique to this question:

Activity 1 has an optimistic estimate of 7 days, a most likely estimate of 16 days and a pessimistic estimate of 25 days. What is the approximate calculated probability that the activity will be finished in 10 to 22 days?

 A. 95.4%
 B. 50.0%
 C. 99.7%
 D. 68.3%

Based on your question, the following are given.
 
O = 7days
 M = 16 days
 P = 25 days
 
Required: approximate probability that the activity will be finished in 10 to 22 days.
 
Solution:
 
1. 1st, let us calculate the PERT estimated mean duration: that is using the formula PERT (µ) = (O+4M+P)/6
 This will be : (7 + 4*16 + 25)/6 = 16 days
 
2. Calculate for the Standard Deviation (single activity) that is using the formula Standard Deviation (σ) = (P-O)/6; Thus (25-7)/6 = 3.
 
3. Now, refering to the normal distribution chart in my first post, figuring out within which standard deviation should 10 to 22 days fall. Based on the chart, below are the probabilities:
 

Follow this link for the detailed chart and explanation: http://www.pmptrend.com/index.php/en/forum/11-Questions-Answers-Queries-Infos-Etc/59-Probability-Calculation-Technique#61

 

Forums: 
PMP Exam Tips, Tricks and Lessons Learnt.
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Aegis747

Fri, 04/27/2012 - 06:21

Hello projmanpro


Thankyou for explaining this question, On trying to "understand this" and your conlusion.. I understand about processing the PERT and Standard Deviation formula's to give you both Pert of 16 Days & Standard Deviation of 3.  But I do not see how you use your chart in the link to come up with the correct answer.. from what I can see here is that the correct answer to this question is C: 99.73%.. This is not based on looking at your chart.. but from simply looking at the 1 Sigma = 68.72%, 2 Sigma = 95.45%, 3 Sigma = 99.73%.. I also guess that the Pert analysis that gives us 16 days, is this a quality check to see of the O, ML & P values are within the 10 - 22 days?


Can you please correct me if I am wrong in regards to the correct answer being C: 99.73% and that the Pert analysis is used to confirm that the O, ML & P is within the 10 - 22 days.. really hoping ou can help with this.. as we are a group of people studying for the PMP exam and we all see your post as a good discuss topic .. hoping you can make this a little clearer for us....

projmanpro

Fri, 04/27/2012 - 12:27

Hi aegis, the 1sigma, 2sigma, and 3sigma are just the same as the sum of the values for each whole band in the normal distribution chart. Note that the chart only explains how did we have that 1sigma, 2 sigma and 3 sigma. If you will notice in the PMBOK, the values of these sigmas are just given without DETAILED EXPLANATION how these sigmas are derived, and for this - the chart of normal distribution explains.

Im sorry to say, your choice which is C = 99.7% is wrong, and i dont wonder why it is wrong because you are giving your answer without having a correct calculation, as u said, by just seeing the values of the sigmas. Even if you solve this question correctly, your answer must be A = 95.4%.

If u have time, please read also the remaining post i did in the same thread, there are other sample questions posted for which i also explained in detail. Probably you can understand more.

this link for example : http://www.pmptrend.com/index.php/en/forum/11-Questions-Answers-Queries-Infos-Etc/59-Probability-Calculation-Technique#61