Theory of Constraints Handbook - James Cox Iii [55]
Tracking Buffer Consumption
To calculate buffer consumption, the PM must have current information on every task that has been started and has not been completed. At each checkpoint (daily or once or twice a week), each project staff member currently working on a task should be asked for the amount of time remaining to complete the task. It is unproductive, for project management purposes, to ask for a completion date or percentage of the work that has been completed. (Historically, “percent complete” has often been overestimated.) A “remaining” time estimate is necessary for the PM to know if action is warranted. The remaining time, added to the time elapsed since the task was started, can be compared to the original estimated aggressive time to determine the buffer penetration or recovery. The reported remaining time changes (i.e. is not always decreasing) each time a query is made.
A task duration overage, meaning the task will complete some time beyond the reduced estimated (aggressive) duration, can be calculated as follows: For a task that has been started and not completed, add the amount of time remaining to complete the task (provided by the assigned resource) to the time elapsed since the task was initiated and compare the current expected total duration to the original estimated aggressive duration. If the current duration is greater than the estimated aggressive duration, the difference between the two is the amount of overage that must be reflected in the appropriate buffer as “used.”23
The overage calculation is not based on when the task was originally planned to start. There is no concern about “start dates” or “finish dates” as each activity time is calculated only on its own planned duration. More will be discussed on this matter later, but as we have already intimated in the preceding section “Communication Plan,” start dates are not emphasized. Instead, CC concentrates on task durations and provides notifications of impending work for each resource on the Critical Chain and for each task without a predecessor task. Otherwise, work is performed in the order in which it arrived in a resource’s queue.
If the overage task is on a feeding chain, the amount of the estimated overage is subtracted from the feeding buffer. If, at some point, a feeding buffer becomes fully consumed, then any remaining overage is shown as utilized in the project buffer. For any task on the Critical Chain, the overage must be subtracted from the project buffer. In a multi-project environment, the project management office (or the equivalent function) should track the performance of the SR (see Fig. 3-8) so that scheduling buffers can be adjusted if the SR indicates a shorter or longer than planned duration for one of its assigned tasks.
FIGURE 3-9 Buffer variation areas. (Reprinted with permission from A Practical Guide to Earned Value Project Management, 2nd ed., Charles I. Budd and Charlene S. Budd. © 2010 by Management Concepts, Inc. All rights reserved.)
Knowing When to Act
PMs need to have a meaningful knowledge about their project’s status and they need to know when to take corrective actions. The amount of buffer utilization provides the required information. Buffers are generally divided into three equal sections of time that can be thought of as “expected variation,” “normal variation,” and “abnormal variation.” They are somewhat analogous to the green, yellow, and red of a traffic control light. An illustration of this division is shown in Fig. 3-9. In Fig. 3-6, there are 30 days in the project buffer, which means there would be about 10 days in each of the buffer variation sections.24
Expected Variation (Green Zone)
Time has been aggregated in the CC buffers to protect the completion date of the project. If everything works according to the CC schedule, some or all of the buffers will be used and the project will complete on or before the scheduled date. As the project work proceeds, we can expect one-third of the buffers to be utilized due to inherent task uncertainty. That means that, in our