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as a single mega-project. Resource 3 is used by activities C1, C2, and C3. Typical PERT/CPM planning calculates the critical path for project 1 to be 23 periods and the critical path for project 2 to be 35 periods. There are three possible orderings for the use of resource 3 by the three activities: C2-C1-C3 (solution 1 designated S1); C1-C3-C2 (solution 2 designated S2); and, C1-C2-C3 (solution 3 designated S3). Any of the three possible solutions—S1, S2, or S3—will delay the completion of the critical path of at least one of the two projects. In fact, solution 3 (C1-C2-C3) will delay the completion of both projects. Additionally, one can imagine the effects of multitasking or job splitting. Although PERT/CPM assumes that an activity, once started, cannot be stopped and restarted, it is common in practice for resource managers to do just that in order to appease various project managers. PERT/CPM does not provide guidelines on when and how to multitask, a common practice in industry.

FIGURE 2-4 Seven problems with PERT/CPM management of single projects identified by Walker (1998).

Cause: Priority of resource use across projects may affect on-time project completion.

Cause: PERT/CPM does not provide priority rules to support project completion.

These are addressed by Guidelines VI, VIII, and XII.

Problem 3: Resource Contention across Projects Caused by Variability of Other Resources Figure 2-4 Problem 3 shows two simple projects diagramed as a single mega-project. Each project has only two activities. Only one activity (activity N in project 2) has any associated variability. Resource 1 is used by activity L1 in project 1 and by activity L2 in project 2 in immediate succession. Typical PERT/CPM calculations estimate the completion date of project 1 to be period 8, and the completion date of project 2 to be period 8. The dashed arrow in Fig. 2-4 Problem 3 shows the order of use of resource 1 as activity L1 then activity L2.

If all possible combinations of activity duration are enumerated, the completion date of project 1 is unaffected by the activity duration variability. However, when activity duration variability results in shorter than expected duration of activity N, a resource contention problem between activity L1 and activity L2 causes the delay of the completion date of project 2. Resource 1 is still in use by activity L1 when activity N is completed in period 2 (its optimistic estimate). As activity L2 cannot begin until activity L1 is completed, project 2 is unable to take advantage of an optimistic completion. A resource contention problem does not exist when activity N is completed in its pessimistic estimate. Therefore, only the late (pessimistic) duration time is added to the enumerated total, and the completion date of project 2 is later than planned. PERT/CPM does not recognize the impact of statistical fluctuation and dependent events on project completion. It should provide guidelines on buffering resources and paths against statistical fluctuations.

Cause: Murphy exists.

Cause: PERT/CPM does not protect against Murphy.

Cause: PERT/CPM does not recognize that multiple projects are interrelated due to the shared use of common resources.

These are addressed by Guidelines VI, VIII, and XII.

Problem 4: Resource Contention across Projects Caused by Variability of Common or Other Resources In Fig. 2-4 Problem 4, two simple projects are diagrammed as a single mega-project. The activity durations in project 1 are variable, while the activity durations in project 2 are deterministic. Resource X is used by activity X1 in project 1 and by activity X2 in project 2 in immediate succession. Typical PERT/CPM calculations estimate the completion date of project 1 to be period 8, and the completion date of project 2 to be period 12. The dashed arrow in Fig. 2-4 Problem 4 shows the order of use of resource 1 as activity X1 then activity X2.

If all possible combinations of activity durations for the two projects are enumerated, the completion of project 1 is unaffected by the activity duration