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Critical Chain
Project Management
(CCPM) | | |
 Project Buffer
The project buffer, located at the end of the critical chain, establishes the target due date for the
project. It also serves as a
tool for project performance measurement and control. The project buffer is the sum of two components;
the variational
component and the bias component.
» See also: Cost Buffer
 Variation (SSQ)
Size the variation component of the Project Buffer using a method to estimate common-cause variation.
You may use
PERT, Monte-Carlo, the square root of the sum of the squares (SSQ) method, or use Goldratt's rule of
thumb: 50% of the
duration of the critical chain tasks (not including gaps). If you use the last method, you do not need
to add the bias
correction.
Bias (> 25%)
Include a bias correction when necessary. Use the bias correction to assure that the total Project Buffer
is at lest 25% of
the critical chain task length. Additional causes of bias include more than six merging paths, recursive
tasks, work
omission (oversight), rework (errors), queuing delay of resources applied to multiple projects, and
incomplete behavior
change to roadrunner behavior (e.g. date-driven or student syndrome).
Action Thresholds
Establish action thresholds to plan and activate plans to recover project duration based on Project
Buffer and Feeding
Buffer penetration. The simple rule is action thresholds at 1/3 and 2/3 of buffer penetration. You may
wish to use a
dynamic threshold for the Project Buffer; i.e. thresholds that increase along the project duration.
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