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Why 6?

4 replies [Last post]
Wed, 2006-04-12 12:59
Hemant Shewnarain
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Hi,

just started to learn about Project Managment and reading about PERT/CPM.

why do u divide by 6? and why do u multiply by 4?

Expected Time:
t = (a + 4m + b)/6
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Replies

Wed, 2006-04-19 10:23
#1
pmkb .
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It’s a weighted average. The 4 and 6 are related to each other in that you have 4 parts of one element and 1 part each of two other elements for 6 total parts. The equation could just as easily have 6/8 instead of 4/6 if the probable estimate was deemed to have a higher weight.

I do not know why 4/6 is generally used as opposed to 6/8, 10/12 or any other weighting factor.

Stacy
Project Management Knowledge Base
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Mon, 2006-04-17 19:54
#2
Clive Randall
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Charlie
Welcomne back
Bell shaped graph
Standard deviation
Not sure higher maths is required
Standard deviation is very simple ask any site engineer who has worked out the results for concrete cube testing.
But why 6 I thought 32 was the answer
Clive
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Sun, 2006-04-16 02:06
#3
Charleston-Joseph...
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Hello,

This is related to the theory of probability.

The graph generated is a bell shape.

The equation is similar to mean average.

I hope you understand and recall your mathematics or higher algebra, differential calculus, intergral calculus, engineering mathtmatics or any subject related to mathematics.

Cheers,

Charlie
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Thu, 2006-04-13 02:18
#4
Steven Oliver
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Where

a = pessimistic duration

4m = 4 * probable duration

b = optimistic duration

generally accepted "rule of thumb". You could equally propose your own method.

(((a+b)/2)+m)/2) for instance

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