Theory of Constraints Handbook - James Cox Iii [37]
Other additions to the diagram are the boxes labeled FB and PCB. These boxes denote feeding buffers and the project completion buffer, respectively. These buffers exist to address Guidelines V, VIII, and IX. Time was taken out of each of the activities in the project, resulting in a.5 probability that each activity will be completed on time. The buffers exist to increase the probability of on-time project completion. The PCB adds time to the end of the project. In this case, since the CC is 20.5 days the PCB would be 10.25 days—the project can then be promised to be delivered in 30.75 days. The feeding buffers exist to protect the CC from variation of non-critical activities. If activities B and E were started on the LS date and suffered any delay, then the CC would be jeopardized. The feeding buffers require that these activities be started sometime before their LS date. (Actual determination of buffer size is left to later chapters.)
FIGURE 2-5 Typical activity-on-node project network with resource contention identified (shading shows same resource use).
In practice, CCPM-SP requires that all activities on the CC be monitored and started as soon as the previous activity ends in order to take advantage of early completions. This process addresses Guideline XI. Additionally, all resources on the CC are monitored to ensure that multitasking is eliminated or minimized to address Guideline V.
Brief Review of Critical Chain Literature
In the book Critical Chain, Goldratt (1997) first published the concept of CCPM. Like several of his prior texts, the book outlined the concept in a narrative fashion and does not seem to have been intended to be a “how-to” manual for CCPM. Rather, its purpose seems to have been to provide a basis for a stream of research that might be pursued by him and others. Pittman (1994) and Walker (1998) examined the single and multiple project environments (respectively) and sought to expose the assumptions and practice of scheduling and controlling projects by traditional methods. Their work provides the basis for the gedankens presented earlier in this chapter.
Hoel and Taylor (1999) sought to provide a method (via simulation) for determining the appropriate size for the buffers required by CCPM. Rand (2000) introduced CCPM to the project management literature framing CCPM as an extension of TOC. He concluded that CCPM not only dealt with the technical aspects of project management (like PERT/CPM) but also that CCPM dealt with how senior management manages human behavior in the construction of the project network as well as the execution of the network. Steyn (2000) followed this research with an investigation of the fundamentals of CCPM. He concluded that a major impediment to implementing CCPM is that it requires a fundamental change in the way project management is approached and that such a change is likely to meet with resistance.
However, Herroelen and Leus (2001) argued that while CCPM was as important to project management as TOC was to production scheduling, CCPM oversimplified the issue of scheduling and rescheduling. Herroelen, Leus, and Demeulemeester (2002) continued much of the same argument in a later paper. Likewise, Raz, Divr, and Barnes re-examined CCPM and concluded that project performance is often a function of the skills and capabilities of project leaders and that “some CCPM principles do make sense in certain situations” (2003, 31). McKay and Morton (1998) as well as Pinto (1999) were concerned that CCPM might be misapplied by managers who failed to understand the underpinnings of CCPM and who attempted to adopt it without fully changing their fundamental approach to the management of projects.
Answering this criticism, Steyn (2002) sought to apply