Theory of Constraints Handbook - James Cox Iii [19]
The primary advantages of Gantt chart scheduling are that it can be easily understood by a wide audience and it provides a visual means to track project progress. The disadvantages are numerous. The chart becomes unwieldy for larger projects (more than about 30 activities) when it extends for more than a single page (or screen, if computerized). The chart does not indicate task dependencies, and therefore fails to communicate how falling behind on one activity might affect other activities. When using a WBS, often there is confusion between defining the WBS and defining the activities of the project. Additionally, as some elements of the WBS might be front- or end-loaded (more work at the beginning or end of the element), the percent progress reported might be over- or understated.
PERT/CPM in the Single Project Environment
CPM and PERT originated in 1957 and 1958, respectively, with CPM examining the tradeoffs between project duration reduction and increases in activity and project costs; and with PERT examining the uncertainty aspects of completion dates for development projects. CPM was originally developed for use with manufacturing plant rebuilds by DuPont and PERT for use with the Polaris nuclear submarine program by the Special Project Office of the Department of the Navy and the consulting firm Booz Allen Hamilton. From their origins to the present, both techniques (and their subsequent merger into one) have been heralded as breakthroughs in managing complex systems.
Once all of the activities are identified (a process that in itself is subject to controversy), a project network can be created. The network organizes the activities in such a way as to clearly show the technological precedence relationships—the simple fact that most activities must be preceded or followed by some other activity or activities. Figure 2-2 shows a typical activity-on-arrow project network. This network has six activities, each having an associated estimate of activity duration. PERT/CPM requires that a forward pass through the network be made to determine the early start (ES) and early finish (EF) times for each activity. Then a backward pass is made through the network using the EF time of the last activity as the late finish (LF) time of the last activity. This backward pass determines the late finish and late start (LS) of each activity. The difference between the LF and EF (or LS and ES) is the slack associated with each activity.
FIGURE 2-2 Typical activity-on-node project network.
Activities having zero slack are called critical activities because any delay in these activities will cause the project to be late. In Fig. 2-1, the critical activities are A, D, E, and F. These activities form what is referred to as the critical path. The primary advantages of PERT/CPM over Gantt charts are that technological precedence (activity dependency) is readily apparent and that it is relatively easy to determine how falling behind on one activity might affect other activities. Activities B and C (Fig. 2-2) have a calculated slack value of four. Therefore, if either of these activities is delayed by more than four days, then the critical path will be negatively impacted because both activities C and E must be completed before activity F can be started. The primary disadvantage of this method is the assumption of readily available capacity on the required resources. Much of the research that follows expands upon this basic premise.
Brief Review of Project Management Literature
The project management literature is enormous—several thousand articles and dozens of books. While project management has grown significantly, many of the problems initially identified almost five decades ago in both the macro-view (applications) literature and in the micro-view (or theoretical) literature still exist today. This literature review provides only a brief glimpse of the continuing themes of project management research since the late 1950s to today. The purpose is to show that the problems of project management