(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?
(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.
Hi,
I have searched but cannot find the answer to this question.
I have produced a programme using P6 vs.7, and dumped the Early vs Late data into excel, to produce early + late curves.
Now I would like to produce a P50 (50% float) curve, however I am unsure how to calculate this as a horizontal split instead of vertical split.
Vertical split calculation is : (Early Start Units + Late Start Units)/2. However this is based on units/vertical split curve.
It should be based on the duration/horizontal axis to give a true P50 representation.
Is there a formula I can use to get this data?
Many thanks
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From a deterministic schedule you cannot determine float distribution probabilities. You must use some modeling to iterate hundreds or thousand times your schedule with varying duration and risk distributions. Then the software will record for each activity the resulting dates as well as float values for each iteration and from this data it will generate the cumulative distribution curves.
Originally when PERT was in its infancy the developers thought that the cumulative probability curve could be obtained from a fixed critical path, the deterministic schedule critical path. They soon found it was leaving out some occurrences of other critical paths and corrected the math using Monte Carlo Simulation to generate the risk events and run the iteration, then again using Monte Carlo would generate the risk events and run again the schedule iteration.
http://hkarms.myftp.org/web_resources/Conference_Presentation/CPM_PERT_Schde_RIsk_Anlys_Consttn.pdf
Note that in the prior reference the probabilities of success from a traditional PERT computation differ from the Monte Carlo simulation. Perhaps those who adopted names similar to PERT in their risk software did not realized that PERT was at some point in time associated with a wrong assumption, that near critical path do not matter. That float is always free; but it comes at some cost in your probabilities of success.
I never liked the name PERT because it tends to forget a lesson no one doing risk analysis shall forget. The new name Oracle is adopting without reference to PERT I welcome.
From: http://en.wikipedia.org/wiki/Monte_Carlo_method
Monte Carlo methods vary, but tend to follow a particular pattern:
The interrelationships on a CPM schedule are so complicated that is only through simulation these curves can be correctly generated.
Usually the float cummulative distribution curve is an output, not an input in traditional schedule risk software.
Hi Sam,
Off the top of my head, I don't think there is a simple formula that will give you what you are looking for. Why not? There are simply too many combinations and permuations that can impact the float.
Again, just off the top of my head, the way I would approach it is to run a simulation (using excel, Pertmaster, @Risk etc) then from the simulation results, find the mean project duration, and the sigma of the data. Then +/- 0.675 sigma from the mean will give you the area under the curve = 50%.
The difference between the simulated early dates at -0.675 sigma and the simulated late dates at +0.675 sigma should give you the float values at P50.
Try it on a simple 10 activity network first and as your problem is a very interesting one, I would be curious to see the results.
BR,
Dr. PDG, Jakarta, Indonesia
http://www.build-project-management-competency.com
Email: PPadmin@planningplanet.com
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