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path is therefore A-B taking 10 periods. However, when all possible durations of each activity are enumerated, the expected duration of the project is not 10 periods but rather it is 10.725 periods. Van Slyke (1963) and later Schonberger (1981) suggested that near-critical paths be managed to ensure that variability on these paths does not affect the critical path. It is interesting to note that had the two paths not converged, the variability of the non-critical path would not have affected the critical path; therefore, this is a special type of convergence problem. (The assumptions of PERT include that a project has only one exit node; therefore, any project with more than one path must have a point of convergence.) The ultimate cause of the delay of project completion is the intersection of a non-critical path (C-D) with the critical path (A-B) when high activity duration variability exists on the non-critical path.

Cause: Murphy exists.

Cause: PERT/CPM does not protect against Murphy.

These are addressed by Guideline VIII.

Problem 3: Scheduling to Time Rather Than the Completion of the Prior Activity The managerial practice of scheduling to time rather than the completion of the prior activity is also affected by activity duration variability. Figure 2-3 Problem 3 shows a simple three-activity PERT/CPM network. In practice, the typical PERT/CPM project manager generates and distributes to the resource managers a written or computer-generated schedule of planned activity start times for that resource manager’s resource based upon the expected duration of the preceding activities. Given that the expected durations of activities A, B, and C are 4, 4, and 4, respectively, a typical PERT/CPM schedule would be as follows:

If, however, all possible combinations of activity duration are enumerated and the activities are started on the scheduled start date (or later if the preceding activity has not been completed), the project will have an actual expected duration of 13.11 periods. Project managers fail to take advantage of favorable completion times when the project is managed according to the above schedule. It should be noted here that optimistic completion times are leveraged only by the last activity in the network since there are no other activities planned to follow it. This means that the managerial practice of scheduling to time rather than completion of the previous activity is magnified in larger projects. The core driver, activity duration variability, is a fact. The practice of traditional project management to schedule to time instead of completion of preceding activity eliminates the opportunity to take advantage of optimistic completions of activities and thus produces poor project results.

Cause: PERT/CPM does not recognize that some resources might be required for more than one activity.

Cause: PERT/CPM provides resource schedules based only on technological relationships and time estimates.

These are addressed by Guideline XI.

Problem 4: Increasing Planned Activity Times Resource managers (as opposed to project managers) have long felt pressure to complete their activity within the expected activity duration estimate. Resource managers then often increase the estimate of activity duration that they submit to the project manager to ensure that the activity is completed on time and to show high utilization of their resources. Low utilization of resources translates into excess resources that will be trimmed. Figure 2-3 Problem 4 shows a simple two-activity PERT/CPM project. The upper project is the network that would be developed if the resource managers were to submit estimates of activity duration based on the actual estimates of activity duration. In the upper network, the PERT/CPM expected project duration would be 12 periods. The lower network is the PERT/CPM project network that the project manager would construct if each resource manager were to increase the expected duration of his activity by 25 percent.

Since the project manager constructs the project schedule based on the