Critical Chain Method
Although the Critical Chain Method is not highlighted too much in the PMP® examination, it is a useful technique to be aware of. Unlike the Critical Path Method, this technique focuses very much on resource availability and seeks to improve productivity where possible.
This is needed because some workers tend not to work at a consistent rate. For example, many under-graduates spend the bulk of their university careers having a good time and then cramming before the exams. If they actually graduate, they tend to bring this behaviour to the workplace. So common is this phenomenon that it has gained a name: “Student Syndrome”.
Another concern for project planners is “Parkinson’s Law” which states that “work expands so as to fill the time available for its completion”. If your people are working to a schedule and they find they finish early, there is a tendency to put their feet up rather than rush off to the next task.
Why these tendencies are a problem relates to the way we estimate activities in a project. If we use three-point estimation or PERT (Program Evaluation and Review Technique) for instance, the estimate given for any given task includes a measure of contingency. (You will definitely see PERT on the PMP® exam!!) If the activity goes well, it should finish early. However, the "students" will wait until the last minute before starting and the "Parkinson's Group" will stretch the work out until the deadline. Thus, the safety margin built into the project schedule gets used up and will not be available when some genuine project risk materializes.
Critical Chaining addresses this problem by taking the contingency out of the activities and creating buffers in the schedule to accommodate likely overruns. Thus the project team is given much tighter deadlines to work to. Three types of buffers are used:
- Project Buffer: This is placed at the end of the project plan as contingency for the critical chain activities. The critical chain is similar to the critical path, but with added attention to resource constraints and dependencies.
- Feeding Buffer: These are added to all the non-critical chains. They increase the length of these paths to equal the critical one.
- Resource Buffer: These are set alongside of the critical chain to ensure that the appropriate people are available to work on the critical chain tasks. Essentially it’s a resource calendar.
To implement this scheme, the project manager prepares the schedule using the Critical Path Method (as studied in detail in the PMP® exam preparation course). Then three steps are taken to extract the contingency into buffers and re-schedule:
- Remove the contingency from the schedule. This exercise is the reverse of what you used to calculate the contingency in the first place. If a percentage was added to the original estimate, remove it; if techniques like PERT are used, replace the calculated estimate with the optimistic one.
- Revise the schedule so that activities are aligned with the late finish date. Also remove resource constraints. If people are assigned to tasks on different chains, they should give priority to those on the critical chain.
- Add feeder buffers to the non-critical paths, making them the same lengths as the critical one. Also add a project buffer to the end of the critical path. It is recommended that this be half as big as the contingency taken out of the critical path. The reduced contingency is to encourage more productivity. It also serves to reduce the overall schedule.
When using the Critical Path Method, problems often delay the schedule, but early completions rarely are published and rarely benefit the project. By making the planned contingency explicit, the project team is encouraged to get straightforward tasks completed quickly and any delays have to be flagged in order to access the buffers. Thus, Critical Chaining aims at a much more aggressive schedule than Critical Path, with less contingency built in. The advantage from the project manager’s point of view is that the use of contingency is visible and its use needs to be justified.