Phillip EdwardsTechnology Program Manager| Built Pty LtdAllambie Heights, New South Wales, Australia
I'm using PERT for my estimates which requires values for most likely, optimistic and pessimistic. I'm wondering how best to calculate optimistic and pessimistic values. According to https://www.project-management-prepcast.co...mate-costs#2928 I can apply a formula to the most likely estimate based on the amount of detail that has gone into the estimate.
Estimate Type Optimistic % Pessimistic %
Rough Order of Magnitude 25 75
Preliminary Estimate 15 50
Budget Estimate 10 25
Definitive Estimate 5 10
So I've tagged each most likely estimate with one of these estimate types based on the amount of detail that was in the original requirement. I then apply the appropriate % to the most likely estimate to get optimistic and pessimistic values.
Problem is I can't find anywhere else that corroborates these factors. Does anyone know if there's an official PMBOK set of factors to use? Saving Changes...
Sort By:
RAJESH K LProject Manager, PMP| Bharat Electronics, Bengaluru, IndiaBengaluru, Karnataka, India
Any estimate requires past data (if available) and / or some set of assumptions. The above details mentioned is only an approach/guideline. Saving Changes...
Sergio Luis ConteHelping to create solutions for everyone| Worldwide based OrganizationsBuenos Aires, Argentina
What you stated is the key point in estimation: available information. That is because people like Barry Bohem created the "Cone of Uncertainty" that is applicable in lot of fields and showed a practical experience based on estatistics he took from projects along the years to related available information with the point of the process where you are performing estimation. Saving Changes...
Those ranges relate to overall estimated budget or timeline and wouldn't apply to individual activity estimates.
For the latter, as Rajesh has indicated, you'd leverage historical data and/or expert judgment.
Kiron Saving Changes...
Phillip EdwardsTechnology Program Manager| Built Pty LtdAllambie Heights, New South Wales, Australia
Thanks for everyone's input. I've done some more research and I now realise now that the figures I originally quoted are the same, or very similar, to Steve McCulloch's Cone Of Uncertainty figures. See http://www.construx.com/Thought_Leadership...f_Uncertainty/. As that seems to be well-established practice I've updated the numbers as follows:
