| Q. 13 |
Where do buffers come from and how do we size them? |
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| Ans. |
It is easy to think that buffers work because they are simply "added time" placed at the end of the work tasks. This is not the case. CCPM plans are almost always shorter in duration—including the buffer times—than conventional plans. The time for buffers is created by removing the inherent slack time that has been built into individual task estimates. When task estimates are given for completing work, these are nearly always based on a high-confidence duration for the task—perhaps 80-90% certainty that it will be completed in the time. This is particularly true in organizations where people are measured or managed based on completing their tasks on time. By moving much of this time from the individual tasks to the buffers, more than adequate time can be "found" to provide the buffers, and still have something left over for reducing the overall project duration. Buffers should never be less than 25% of the time of the critical chain and are typically sized between 30% and 50% of the critical chain duration. |
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