Theory of Constraints Handbook - James Cox Iii [56]
Normal Variation (Yellow Zone)
The basis for Deming’s discussion of tampering was his hypothesis (now universally accepted) that there are two kinds of variation in any process. He called them “common cause” variation and “special cause” variation (Deming, 1986). Common cause variation is inherent in the design of the process itself because no process is perfect. By their very nature, project task times are uncertain. The utilization of the second third of CC buffers is usually caused due to the inherent uncertainty of task duration prediction. Small variations in the operation of a project are not a reason for alarm, but if the second third of the buffer begins to be used to cover task overages, plans should be formulated to recover lost time. However, to avoid tampering, action should not be initiated until abnormal variation, the last third of the buffer, is experienced.
Utilization of the last (abnormal variation or red) section of a buffer is usually the result of special cause variation and it is wise to observe the scout motto to “be prepared.” The time for the PM to develop an action plan to be used if the red (abnormal) section of the buffer is penetrated is before it happens—while only the second section of the buffer (normal variation) has been penetrated. Among other possibilities, an action plan might include such items as arranging for the possible use of overtime or additional resources, outsourcing parts of the project, or securing an agreement to reduce the scope of the project.
Abnormal Variation (Red Zone)
Special cause (abnormal) variation is usually the result of a unique event outside of the normal course of the project operation. Such events could be as simple as illness of a project resource or as momentous as a natural disaster. When the red portion of the buffer is penetrated, it is definitely time for action and the implementation of the plans made while buffer consumption was in the middle section of the buffer.
If a feeding buffer is involved, the appropriate action is to carefully monitor the project buffer. If the project buffer still holds adequate safety, immediate action may not be necessary. If the project buffer is involved, the action plan should be initiated immediately. If a scheduling buffer is involved, the initiation of the next project should be delayed if possible. Some precedent tasks of the next project may have already been started before knowledge of the SR problem surfaced. If the next project has already been initiated, it would be prudent to delay the initiation of other projects that occur later in the project schedule.
Adjusting Buffers
As a project nears completion, it is expected that some or all of the buffers will be utilized. It is less and less important to maintain the full size of the protective buffers unless they are needed. Remembering that we had to add 4 days from a feeding buffer, the sample project in Fig. 3-6 (or Fig. 3-5) starts with 30 days in the project buffer. Compared to the original CC time, that is a ratio of 30/52 ≈ 0.58. This ratio of task duration to buffer time should be maintained throughout the performance of the project.
Using Fig. 3-6, for example, when Tasks A, D, and E have been completed, Tasks B, C, and J on the Critical Chain would leave 38 days of work to be completed. Maintaining the same ratio of 0.58 means that the project buffer can now be adjusted down by 8 days to about 22 (actually 22.04). The traffic light buffer sections of the new buffer would be divided into thirds of 7-1/3 days each and an adjustment made to calculate the new action triggering points. The amount of the reduction in the buffer (8 days) is subtracted from any previous buffer utilization and the difference