Theory of Constraints Handbook - James Cox Iii [46]
Not infrequently, these resources think they are improving the quality of their product by adding “extras” not included in the original specifications for the task. (In our experience, this is especially true on software projects.) However, the unspecified and undocumented addition may cause problems, sometimes major problems, further along in the project.
Management rarely distinguishes between task uncertainty and the time that is lost when tasks are started late, constantly interrupted, or when workers fail to turn over finished work. CC acknowledges these dysfunctional behaviors and establishes policies to deter their occurrence. The next section summarizes the basic elements of CC.
Key Elements of Critical Chain
While many of the basic project management concepts are preserved in CCPM, it is designed to overcome the most egregious issues that have resulted in the poor performance of projects as described in the previous chapter and in all-too-familiar press reports. The magnitude of change required demands a different approach. When people are doing their best and outcomes are unacceptable, as Deming (1993, 172–175) so strongly advised, we must change the system.
Changes are required in planning, scheduling in single and multi-project environments, and in managing the project.
Issues in Creating a Project Plan
Most stakeholders involved in a project are quite familiar with the general requirements of the project that include issues such as identifying the project objective, having a project charter, understanding the work breakdown structure, acquiring resources, and creating a plan for the budget and scheduled tasks.3 Once planned, most project management books suggest that the critical path, the longest chain of dependent tasks, is the most important in project completion. Therefore, this path is given preferential treatment when assigning scarce resources.
When planning a CC project, the total budget may be the same, but there are particular scheduling requirements that differ from the traditional critical path approach. However, we will discuss the scheduling differences then return to the project budget toward the end of this chapter.
Task Duration Estimates
Human resources naturally include safety time in their duration estimates. In defining a CC schedule, this safety is removed from individual (local) tasks and aggregated to protect the entire project. It can be helpful if the PM has some historical knowledge of an individual resource’s safety preferences. In general, about half of a task’s “safe” time, the time required to be 90 to 95 percent confident of task completion, is there to cover interruptions, surprise rework, urgent unanticipated assignments, and task estimation error.
Rather than providing “start” and “finish” times for every task, as recommended by traditional project management, CC uses task durations and asks resources to work on a first-in, first-out (FIFO) basis for all queued tasks. Start times are provided only for initial activities on a path—those with successor activities but no predecessors.
Task Uncertainty
Just as a management reserve is established to cover the uncertainty of estimated costs, task uncertainty is managed in CC with buffers of time. Besides referring to these blocks of time with no scheduled activities as buffers, some U.S. government guides call them schedule reserves or schedule margins.4 (For example, see NASA, 2009, 223–224; United States Government Accountability Office [GAO], 2009, 56, respectively, for NASA and GAO best-practices remarks).
Buffers will be explained more fully later and will be illustrated in an example of a project scheduled using CC concepts.
Resource Contention
In most traditional project plans, encountering resource unavailability or tasks delivered late can cause the critical path to shift. Some projects will have the critical path shift several times during project execution. These shifts result in constantly