Theory of Constraints Handbook - James Cox Iii [35]
However, augmenting the activity duration estimates has caused the planned completion date of each project to be later than would be the case without increasing activity duration estimates. Both of the individual project completions have been extended for minimal or no gain in probability of on-time completion.
In this case, the project manager receives praise for completing the projects ahead of schedule and the activity managers receive praise for completing their respective activities ahead of schedule (though only 67 percent of the time). If the estimates of activity duration had not been increased and the project manager had planned to time (as in Fig. 2-4 Problem 6), the expected completion date of project 1 would have been 9.5 periods and the expected completion date of project 2 would have been 7.625 periods. Clearly, this is a better result than 10 periods and 8 periods (projects 1 and 2, respectively) both in time and in cost, but the project manager would be punished for failing to meet the planned completion date. Additionally, had the activity duration estimates not been increased and the activities had been planned to completion, the expected completion dates of projects 1 and 2 would have been the same as their respective planned completion dates. PERT/CPM does not discuss the impact of overestimating activity times across projects on project completion.
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, X, and XII.
The Use of PERT/CPM Critical Paths in the Single Project Environment
In conducting research on project management using simulation as compared to using project management in practice, some differences are noted. In most simulation models, the succeeding activity is linked to the completion of the proceeding activity (scheduling by activity completion). For example, if activity A was scheduled to finish at time 10 but finished early at time 7, then activity B started at time 7 instead of waiting until the scheduled start time of 10. This seems to be the common practice in conducting research on project management.
In practice, however, with the growing use of project management software (Krakow, 1985; Lowery and Stover, 2001), the convention is to schedule by time (scheduling by time). Each resource is given a project schedule indicating when the resource is to start a given activity and how long it is to last. Where software is not used, either convention applies. Seldom, however, can a resource immediately reschedule what it is doing to start an activity early unless given warning.
The point here is that research does not simulate reality in its simplest form. In practice, where projects are large, several functions are involved, and project management software is used, projects seldom benefit from optimistic (early) completions. This means that the project is assured of being late unless extraordinary actions are taken to keep the project on schedule. If activities are only completed in their mean and pessimistic times, then activity times and project times are consistently understated. The project will always be late.1
Cause: Theory does not support practice.
This is addressed by Guideline XI.
The Use of PERT/CPM Critical Paths in the Multiple Project Environment
Two approaches are recommended in the research—the use of