Theory of Constraints Handbook - James Cox Iii [30]
If the project manager schedules to time (a resource schedule) rather than the completion of the prior activity, the actual expected duration of the project is 13.33 periods. In this case, the project manager receives praise for completing the project ahead of schedule and the activity managers receive praise for completing their respective activities ahead of schedule (although only 67 percent of the time). If the estimates of activity duration had not been increased and the project manager had planned to time, the actual expected duration of the project would have been 12.67 periods. Clearly, this is a better result than 13.33 periods in both duration and cost, but the project manager would be punished for failing to meet the scheduled completion date. Finally, if the estimates of activity duration had not been increased and the project manager had scheduled to completion of the prior activity, the actual expected duration of the project would have been 12 periods. Again, the ultimate causes of project delay are resource managers including local protection in activity times and the project management practice of scheduling activity start times based on the expected time estimates instead of scheduling activities to start based on the actual completion of the preceding activity when variability exists.
Cause: Murphy exists.
Cause: Resource managers are expected to finish activities when planned.
Cause: Resource managers do what they feel is necessary to ensure resource utilization and that the resources are available when promised.
These are addressed by Guidelines IV and X.
Problem 5: Early Consumption of Path Slack Figure 2-3 Problem 5 shows a simple PERT/CPM network. There are two paths in the network: A-C-E taking 28 periods and B-D-E taking 33 periods. The slack associated with the non-critical path is therefore 5 periods. Since activity E is critical (0 slack), all of the slack associated with the non-critical path can be “assigned” to activities A and C. Because a non-critical path has slack, a typical PERT/CPM project manager would assign a start date of 5 to activity A. The expected finish date for activity C would therefore be period 18. If one examines the portion of the critical path before activity E (namely B-D), it is obvious that the expected finish date for path B-D is also period 18. It should be clear from the example of variability and convergence points that if activity duration variability exists in this network, then activity E cannot be expected to start in period 18. Consequently, the actual expected duration of the project cannot be 33 periods, but rather it must be longer. Two problems exist in the practice of project management. First, all of the slack associated with the non-critical path was absorbed in the planning stage of the project. PERT/CPM treats path slack as if it is associated with a specific activity and provides little recognition that once consumed by early activities, it is not available for protection for later activity (it is called activity slack, not path slack). Second, the project is delayed because of scheduling activity start times based on the PERT/CPM-calculated late start date rather than scheduling activities to start based on the actual completion of the preceding activity when variability exists.
Cause: Murphy exists.
Cause: The projects is a major undertaking, which determines the success or profitability (goal) of the organization.
Cause: Project managers delay expenses by starting activities as late as possible.
These are addressed by Guidelines I, II, III, and IX.
Problem 6: Resource Contention Many researchers have recognized that the PERT/CPM assumption of infinite capacity does not accurately reflect the reality of finite capacity (e.g., Davis, 1966; 1973; Westney, 1991; Badiru, 1992; Davis et al., 1992; Dean, Denzler, and Watkins, 1992; Pittman, 1994; Zhan, 1994). When