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Unit 9: CPM /PERT

Project Buffers Notes

The project buffer is a time buffer placed at the end of the critical chain to protect the overall
project schedule. Its size can be calculated as the square root of the sum of the squared differences
between the original task duration estimate and the reduced task duration estimate.
Feeding Buffers

The feeding buffer is a time buffer placed at the end of a sequence of tasks that lead into the critical
chain. Its size is calculated the same way as the project buffer size.

Resource Buffers

The resource buffer is different than the previous buffers. First, it is not a time buffer. It is a flag,
usually placed on the critical chain to alert a resource that it is needed. The flag can be placed at
intervals such as one week before the resource is needed, three days before the resource is needed,
or one day before the resource is needed. Because it does not contain any time interval, it does not
affect the project scheduled completion date. It serves merely to protect the critical chain.

9.4.2 Using Buffers

Let’s return to the example project in Figure 9.4. So far, the project can be completed at the end
of the eighth day. Resolving the resource conflict with Mukesh did not add any duration to the
project. First, we put a project buffer after C3, the final task in this project. Its size is the square
root of the square of the contingencies in tasks C1, C2, and C3 (or the square root of 9 + 9 + 4),
which is approximately 4.7 days. Next, we add feeding buffers at the end of the sequences A1-A2
and B1-B2. The calculated buffer sizes are 3.6 days for A1-A2 and 2.8 days for B1-B2. Incorporating
these buffers into Figure 9.4 results in Figure 9.6.

Figure 9.6: Project Schedule with Buffers Added

Note that the CCPM approach gives us a project duration (including contingency buffers) of 13.7
days, compared to the TPM approach of 16 days. In other words, extracting the contingency from
each task, collecting it at the end of the task sequences, and managing it has saved us an average
of 2.3 days out of a 16-day schedule. That’s about a 14 percent schedule improvement.

9.4.3 Managing Buffers

The TPM will see the buffer as nothing more than management reserve. The only real similarity
between a management reserve and a buffer may be in how they are managed. Managing
management reserve and managing buffers can follow very similar logic. The three decision
trigger levels in buffer management are as follows:

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