Theory of Constraints Handbook - James Cox Iii [54]
The initiation of each project (in the predetermined priority order) is scheduled such that the SR is leveled across the projects. That is, the SR’s22 tasks are never overlapped. New projects can be initiated only at the time that the SR’s first task in the new project occurs after the SR’s last task in the current project is scheduled to complete.
In addition, we do not want to schedule the SR’s tasks in the different projects back-to-back in case one of the tasks should overrun its estimated duration. To provide some protection for the overall multi-project schedule, a scheduling buffer is used in each project.
The scheduling buffer is inserted into each project in front of the first task to be performed by the SR. When problems arise in any project, a time buffer in front of the SR’s task in the next project will minimize slippage in the entire portfolio schedule. The size of the buffer is optional, but it should be relatively large. Because our entire project portfolio schedule depends on the scheduling buffer, a general rule is to make the buffer at least as large as the duration of recent task times scheduled in the higher-priority project. This is especially true when first establishing a CC multi-project environment. However, buffer size can depend on experience, individual project configurations, and other factors.
For example, suppose we select Resource 4 as a SR. The last two tasks of Resource 4 in the sample project (see Fig. 3-5 or Fig. 3-6) are scheduled with sequential durations that total 20 days. This project is unusual in that Resource 4 was required to perform four separate tasks on this project. The next priority project only requires Resource 4 to perform two tasks, about the average for this organization. Therefore, the organization has decided that 20 days is a sufficient scheduling buffer to delay the start of Project 2.
Figure 3-8 shows the latter part of the current project, “Project 1,” and two additional projects being initiated as Resource 4 (black color) is available to perform work on them. (Only the latter part of Project 1 and only the beginning part of Project 3 are shown in Fig. 3-8 because the figure is designed to show how the scheduling buffers in Projects 1 and 2 sequence the release of Projects 2 and 3 based on availability of black Resource 4.) The previously described Strategic-Resource-Buffer methodology sufficiently staggers the entry of work into the organization’s system, according to a project’s priority, to avoid, to a great extent, any temptation for resources to multi-task. Should there be an occasional example of a resource being required to perform work on different projects at the same time, the PMO or the resource manager can decide which task should have priority.
FIGURE 3-8 Scheduling resource (black) and scheduling buffers space the entry of new projects.
The CC in each project in Fig. 3-8 is identified with white stars (). Project 2 has black Resource 4 scheduled on two tasks for a total of 25 days. Therefore, the scheduling buffer to sufficiently delay the start of Project 3 is 25 days.
Establishing clear project priorities to support an organization’s strategy is the responsibility of top management. Priorities should be clear and firm. Should a more desirable project opportunity arise, project scheduling can be adjusted. However, the impact of delaying projects already scheduled should be computed and considered carefully prior to adding a new project. Change control at the portfolio level is as important as change control on an individual project.
Project Control: The Power of Buffer Management
We previously discussed the purpose of buffers as a project-planning device to concentrate protection for individual projects and to control the initiation of projects in a multi-project environment. Another very important use of CC buffers is to provide a project management tool so a PM knows when to take action and when to avoid doing so unnecessarily.