Theory of Constraints Handbook - James Cox Iii [57]
FIGURE 3-10 Original and revised buffer sizes (after completion of Tasks A, D, and E).
Sometimes consumption of the buffer is referred to as buffer burn rate. The TOCICO Dictionary defines this term as, “The rate at which the project buffer is being consumed in Critical Chain Project Management. The rate is calculated as the ratio of the percent of penetration into the project buffer and the percent of completion of the Critical Chain” (Sullivan et al., 2007, 7–8). A result of 1.0 would indicate that the original relationship between the Critical Chain and the buffer is being maintained. Using this formula in our sample project, the burn rate would be 0.33 [percentage of buffer consumption: (10 days)/(30 days)], divided by 0.27 [percentage of Critical Chain completed: (14 days)/(52 days)], or 1.22, a bit higher than the desired 1.0. However, Fig. 3-10 indicates the project is still in the expected (green) range of variability.
Feeding buffers similarly are adjusted as the feeding paths are completed. Since Task A has been completed, its feeding buffer is no longer needed. However, 12-day Task A required 16 days for completion, so 2 of the 10 days of the original project buffer used resulted from Task A, which was able to use only 2 days of its original 6 days of feeding buffer.
The PM should know how and why to perform these buffer calculations each time they receive task reports on active tasks, but in complex project settings it would be a very difficult chore without CC project software that reports resized buffers, buffer penetration, and other useful project management information. Various CCPM software programs may compute buffer consumption slightly differently, but the example in Fig. 3-10 will give you an understanding of how the buffers can be adjusted manually as the project is completed.
A typical fever chart showing the trend of buffer consumption versus CC completion over a number of reporting periods, is illustrated in Fig. 3-11. The solid black area (top of Fig. 3-11) represents the red zone, requiring immediate action, the dark grey area in the middle (diagonal) represents the yellow zone, where plans are made, but action is delayed, and the light grey area represents the green zone where things are going well and the PM should not intervene. Note that by the fourth reporting date, buffer consumption jumped to about 80 percent while only 40 percent of the CC was completed. The project was in the red zone and required immediate intervention. While the project recovered (back to the yellow zone) by the sixth period, additional recovery plans should be formulated in order to ensure on-time completion.
FIGURE 3-11 Buffer tracking on a fever chart [adapted from Newbold (2008), 112].
Using Buffer Consumption Information to Continuously Improve
When buffer consumption enters the expected (yellow) variability zone (see Figs. 3-9 and 3-11), every task that overruns its expected (aggressive) time should be analyzed for the cause. This investigation might be initiated for any buffer consumption, starting from the beginning of a project. Causes of overruns (overages) include the following:
Material damaged or of poor quality
Resource ill or absent due to family emergency
Task poorly defined (or poorly understood)
Quality problem with previous work
Resource assigned to a more critical project by the PMO (or similar body)
Subcontractor problems such as poor quality or late delivery
Unexpected event such as abnormal