Creating a viable schedule and adjusting it as necessary throughout the life of the project is an important part of the project manager's responsibilities.
The scheduling cycle (see Figure 17.7.1) begins with the work breakdown structuring process, which subdivides the project's work in to a set of manageable and easily understood tasks. The next step is to estimate the time, man hours, and other resources required to complete each task. Then the relationships between tasks must be documented. Many can be performed in parallel while some must necessarily come before others. In any case, each task's interdependence with every other task must be determined. The project manager is now ready to create the actual schedule, using one of the methods discussed in the next section. Once the schedule is formulated, it must be checked against available resources for feasibility. If the schedule contains periods of time during the project when more resources are called for than will be available, the scheduler must iterate back to an earlier part of the scheduling cycle and make adjustments so that the eventual schedule will be compatible with available resources. Once this is accomplished, the project manager should make a final check to ensure that the proposed schedule will in fact satisfy the project constraints (budget, time, and performance criteria). If constraints are met, the schedule can be finalized.
FIGURE 17.7.1 Project scheduling and estimating cycle.
Scheduling is a continuous process during the life of a project. In the dynamic project environment, changes are always occurring which necessitate replanning and rescheduling. Mechanisms that keep the project plan and schedule valid and current should be put in place at the outset of the project.
Scheduling Methods. The Gantt chart or bar chart (see Figure 17.7.2) was one of the first techniques used by project managers to schedule and control projects, and it is still widely used. It is simple, graphic, easy to construct and change, and convenient for displaying progress. Project task times are usually shown as horizontal bars that begin and end at the scheduled activity start and end times, respectively. However, Gantt charts do not do a good job of displaying task dependencies and interrelationships. Furthermore, the critical path, the set of sequential activities which define the ultimate length of the project, is not obvious (Rosenau, 1992). So, while the Gantt chart is a useful tool for quick overviews and broad statusing of projects, it needs to be augmented by a more comprehensive scheduling tool in all but the most elementary project management situations.
Critical path method (CPM) and program evaluation and review technique (PERT) are two critical path systems developed to address the shortcomings of the Gantt chart, and enable project managers to do a better job scheduling complex projects. They both show project task durations, and indicate the earliest task start times as the Gantt chart does, but they also show the precedence between activities. This allows the project manager to determine the most critical activities and to more easily modify and determine the status of the schedule.
CPM assumes activity durations are known with a reasonable degree of certainty, and are thus useful in industries like construction that have a long history of performing the various tasks to draw from in formulating accurate project activity estimates (e.g., dig basement, pour footer, etc.). On the other hand, PERT is designed to be used in applications like R&D and aerospace wherein many activities are being performed for the first time and there is no history to draw upon in formulating activity duration estimates. PERT employs statistical techniques to assist the scheduler to devise reasonable activity duration estimates and an estimate for the overall project duration.
Figure 17.7.3 depicts a simple CPM network drawn in the activity on node (AON) format. The nodes or circles denote activity. The connecting arrows show precedence between activities. There are various possible routes through the network. The longest route defines the predicted total project duration. The activities on this longest route, or critical path, are called critical activities. Any delay in the execution of these critical activities translates into a delay in overall project completion. Other non-critical activities have a certain amount of leeway (or slack) associated with them which allows the project manager some flexibility in scheduling resources. Thus the project manager should pay special attention to critical activities in making tradeoff decisions, as these are the ones that can potentially extend the project duration. However, it should be noted that non-critical activities can potentially become critical if they experience delays greater than their original slack, so the project manager cannot become complacent about noncritical activities either.
In solving the network, forward and backward passes are made according to the formulas given in Figure 17.7.3. Together these two passes determine the earliest and latest starts and finishes for each activity. Also determined are the project's duration, and the critical path (path that contains activities without slack), which represents the predicted project duration. In the example, activities A, C, D, and F have no slack and are thus critical. The path ACEF is the critical path and defines the project duration, 26 days. Once created, the network can be continually updated and used during the life of the project to assist the project manager to understand current project status and make informed tradeoff decisions.
It is not feasible to solve any but the most simple PERT/CPM networks by hand. Fortunately, software is widely available for modern personal computers which is capable of constructing and solving these networks for small and medium sized projects. Very large project networks require the power of mainframe computers. The Dreger (1992) text presents comprehensive examples using typical personal computer project management software in realistic project situations.
A more detailed exposition of PERT/CPM techniques can be found in Moder et al., 1983.
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What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.