Theory of Constraints Handbook - James Cox Iii [48]
Modifying Task Duration Estimates
Following initial project planning activities (i.e., identifying the project objective, authorizing the project charter, determining required tasks and the work breakdown structure, etc.), a most critical step in preparing a project plan is getting estimates for task durations. In most organizations with a functional or matrix structure,6 project resources are primarily responsible to a line manager and only secondarily to a PM. The resources know that project tasks will be in addition to their usual job responsibilities. They often do not know how much of their time the task will take. They do know that they will be expected to complete their project tasks within the estimated (promised) time.
FIGURE 3-2 Estimated dedicated task times.
If resources could work uninterrupted on a task until it is completed, they probably would provide the estimated durations shown in Fig. 3-2. However, they would be risking their jobs to report these durations to their PM.
Veteran resources (all of whom have experienced unplanned work assignments and interruptions that affect their ability to complete their assigned tasks on time) rely on their intuitive knowledge that the actual task time will be an element of a skewed distribution having some minimum time but possibly a very high maximum. Therefore, knowledgeable resources typically give a task duration estimate that they can expect to meet at least 90 percent of the time. (Recall that resources in traditional project management are held responsible for completing the task by their estimated times.)
Assume, for our simple example, resources provide the task times illustrated in Fig. 3-3 for a traditional, resource-leveled project.7 Task D, on the top path of Fig. 3-3 and Task J at the very end of the project, show the skewed continuous distributions associated with the task estimates of 16 days and 28 days for Tasks D and J, respectively. All of the tasks have similar distributions that justify the times submitted, although they are not shown in the figure. The tasks that comprise the critical path are highlighted with a solid thick grey line from Task D and two-thirds of Task E, to Task B, shifts back to Task E (to complete the final 4 days of the 12-day task) estimate, and then continues to Tasks F, G, and J.
Note that the lower path of activities in Fig. 3-3 is scheduled to begin as soon as possible (immediately after Resource 4 completes Task D), as is the general practice in traditional project scheduling. The generally held erroneous assumption that an early start helps ensure an early finish guarantees path starts as soon as possible. The project is scheduled to complete in 104 workdays. Microsoft Project 2007TM software splits the work on Task E into two parts and includes Task B on the critical path, as reflected in Fig. 3-3.
FIGURE 3-3 Traditional resource-leveled project schedule (showing 2 of 10 distributions).
A major precept of the Theory of Constraints (TOC) states that the sum of local optima is not equal to global optima. In managing a project, the concept implies that concentrating on individual task completion does not ensure that the project will complete on time. The entire project may be in danger of not completing on schedule when even a few tasks are late (especially if they are on the Critical Chain). This means that we should change our focus from individual task completion to project completion. This focus is accomplished in a CC schedule by removing the safety (time) built into the individual tasks and concentrating this safety where it will protect the project’s completion rather than the completion of individual tasks.
Can we really do this and not jeopardize the completion of every task? Yes, but it will take some changes in organizational behavior patterns, which we will discuss later. First, let’s look at the statistics that really indicate that there is little overall danger in removing