In this lesson, you’re expected to learn about:
– the program evaluation and review technique
– critical path method
– work-breakdown structure and gantt charts
Two popular project scheduling techniques include the program evaluation and review technique (PERT) and critical path method (CPM).
Other related tools and techniques include work-breakdown structure (WBS) and Gantt charts.
Program Evaluation and Review Technique (PERT)
Project management frequently uses network diagrams to plan the project, evaluate alternatives, and control large and complex projects toward completion. The program evaluation and review technique (PERT) requires extremely careful plans from the
very outset of the project.
This allows management to allocate resources to critical areas before they become critical. This will alert a manager to trouble areas or bottlenecks before they become a major problem and the source of a project overrun. PERT also helps to allocate resources but has no influence on the excellence of the end product.
PERT improves communication upward to the manager and the customer (client). It lets the supervisor believe that the project manager is doing a superior job, regardless of how well the project manager is actually performing.
When PERT is used on a project, the three time estimates (optimistic, most likely, and pessimistic) are combined to determine the expected duration and the variance for each activity:
1. Optimistic. An estimate of the minimum time an activity will take. This is based on everything “going right the first time.” It can be obtained under unusual, “good-luck” situations.
2. Most likely. An estimate of the normal time an activity will take—a result that would occur most often if the activity could be repeated a number of times under similar circumstances.
3. Pessimistic. An estimate of the maximum time an activity will take—a result that can occur only if unusually “bad luck” is experienced.
The expected times determine the critical path, and the variances for the activities on this path are summed to obtain the duration variance for the project. The expected completion time is computed as:
Expected Completion Time = 1/6 (a + 4m + b)
where a is optimistic time,
m is most likely time, and
b is pessimistic time.
The expected activity times derived from a three-estimate, PERT-type calculation provide a more accurate estimate and allow the activity time variance to be calculated and included in the estimates of project duration.
The critical path method (CPM) is a powerful but basically simple technique for analyzing, planning, and scheduling large, complex projects.
In essence, the tool provides a means of determining which jobs or activities, of the many that comprise a project, are critical in their effect on total project time and how best to schedule all jobs in the project in order to meet a target date at minimum cost. CPM is an extension of PERT.
– What immediately preceded this job?
– What immediately succeeds (follows) this job?
– What can be concurrent with this job?
A delay in a critical job will cause a comparable delay in the project completion time. A project contains at least one contiguous path of critical jobs through the project diagram from beginning to end. Such a path is called a critical path, and there can be only one critical path at a time in a project.
• The project consists of a well-defined collection of jobs or activities that, when completed, mark the end of the project.
• The jobs may be started and stopped independently of each other, within a given sequence.
• The jobs are ordered in a technological sequence (for example, the foundation of a house must be constructed before the walls are erected).
CPM focuses attention on those jobs that are critical to the project time, it provides an easy way to
determine the effects of shortening various jobs in the project, and it enables the project manager to evaluate the costs of a “crash” program.
Crashing a project means reducing its time with increasing cost.
It is physically possible to shorten the time required by critical jobs by assigning more people to the jobs; working overtime; and using different equipment, materials, and technology.
Crash time is the time required by the path if maximum effort and resources are diverted to the task along this path.
A balance can be obtained when a manager knows what the normal time and the crash time would be. The goal is to select the activity on the critical path with the least crash cost per day to crash first.
The work-breakdown structure (WBS) was first intended as the common link between schedules and costs in PERT cost application. Later, it became an important tool for conceptual organization of any project.
The WBS provides the necessary logic and formalization of task statements. The WBS prepares the work packages, which usually represent the lowest division of the end items.
The Gantt chart is used for milestone scheduling where each milestone has a start and completion date. A milestone represents a major activity or task to be accomplished (e.g., a design phase in a computer system development project).
The structure of the chart shows output plotted against units of time. It does not include cost information. It highlights activities over the life of a project and contrasts actual times with projected times using a horizontal (bar) chart.
It gives a quick picture of a project’s progress in knowing the status of actual timelines and projected timelines.
If there is slack time, there is no need to use sophisticated and tight scheduling methods such as PERT.