Theory of Constraints Handbook - James Cox Iii [51]
FIGURE 3-5 Critical Chain project schedule with project and feeding buffers.
In terms of the original 5FS, dealing with merging paths would be the subordination step, Step 3.
Another Look at Resource Contention
In order to develop a project plan that has any chance of on-time completion, we must schedule tasks in such a way that the assigned resource is not scheduled to work on more than one task at a time. In CC scheduling, we typically start tasks as late as possible and, when scheduling manually, schedule shorter tasks toward the end of the project when possible. This usually will result in less resource contention as the rescheduling proceeds and provide better opportunities for time recovery earlier during project execution.
As mentioned previously, the critical path in traditional projects may change many times. In CC scheduling, resolving resource contention is doubly important and the possibility of resource contention must be checked in every step of the process.
Looking at the intermediate project schedules in Fig. 3-4 and Fig. 3-5, we see that Task F and Task G are forced earlier in time by the insertion of a 5-day feeding buffer. However, no new resource contention arises due to the insertion of this buffer. Task I, Resource 3, which was pushed earlier by previous action by the PM, is not affected by the insertion of a 7-day feeding buffer. If work on Task I has not been completed by the time Task B (that precedes Task C) is completed, normally the PM will inform Resource 3 to cease work on Task I and move to Task C on the CC.14 Since Task D is on the CC, Resource 4 first will complete that task, then begin Task H. Should Task D require more than 8 days to complete, Task H might be delayed starting, but the feeding buffer and project buffer can absorb any delays. This simple project example is unusual in that new resource contention does not result from the insertion of feeding buffers. You always should expect new resource contention arising when feeding buffers are added to a project schedule.
Scheduling a resource to work on more than one task at a time can easily result in the resource multitasking in order to show “progress” on all assigned tasks. Making sure this does not occur in a single project, by leveling all resources, avoids this type of unproductive multitasking. Of course, in a multi-project environment, it is impossible to level all resources over all projects with any confidence that resource contention can be avoided. We must use another CC technique, discussed later, to avoid resource contention in a multi-project environment.
Communications—Step 6
It is imperative that a resource that is assigned to a task on the Critical Chain immediately begin that task as soon as the preceding task is completed. CC uses a notification system that informs the next resource that she or he will be required to work on a CC task. This notification is given a brief time interval before the previous CC task has been completed. In the sample project, this time interval would be two or three days at most.
FIGURE 3-6 A complete and fully protected Critical Chain project schedule.
Step 6 of CC project scheduling15 ensures this notification occurs by placing resource buffers in the project schedule at appropriate points. Resource buffers do not have any task time: they are communication tools. In addition, resource buffers should be placed in the project plan to inform resources assigned to tasks with no predecessor when they should begin work. Tasks A and D