Lesson 4: PERT Charts
Lesson Objectives:After reading this lesson, you should be able to:
- Describe the need for and purpose of a PERT chart
- Explain the parts of a PERT chart
- Explain the pros and the cons of using a PERT chart
- Create a PERT chart for a project
Another planning and controlling tool is the Program Evaluation Review Technique (PERT). This tool can also be referred to as the Critical Path Method (CPM). Both PERT and CPM can be used interchangeably, but for the purposes of this lesson, only the PERT term will be utilized.
Primarily, the PERT chart identifies the critical path for the project. The critical path is the sequence of tasks where there is no slack time. In other words, if any task on the critical path takes longer than expected, the end date of the project will be affected. This only applies if there are tasks that can be completed in parallel. For example, all of the tasks in Figure 6 occur sequentially and therefore every task is part of the critical path.
Figure 6: Simple Gantt ChartIn Figure 7, tasks 6 and 7 occur simultaneously and therefore one of these tasks is on the critical path and the other one is not on the critical path.
Figure 7: Project TasksBy inspection, in Figure 7, task 7 takes longer so this task is on the critical path whereas task 6 is not on the critical path. If task 7 takes six days instead of five, it is clear that the end date of the project will be affected. On the other hand, if task 6 takes four days instead of two, there will be no impact to the final completion date of the project.
The PERT chart will also identify the slack time of tasks that are not on the critical path. Any task not on the critical path has some slack time. Referring back to Figure 7, task 6 is estimated to take two days but does not have to be completed until task 7 is completed which is estimated at five days. Therefore, task 6 can take up to five days and still not impact the schedule. Therefore, task 6 has a slack time of three days (five days max and two days estimated). Figure 8 shows a simple PERT chart of the project in Figure 7.
Figure 8: PERT Chart
A simple PERT chart is shown in Figure 9. There are four tasks in this project. Task A takes one day to complete. Task B takes 4 days to complete and can be executed in parallel with Task C which takes 11 days to complete. When tasks B and C are completed, task D can be executed which will take 3 days to complete.
Figure 9: Simple PERT Chart
There are various formats for a PERT chart. Figure 9 was created with a template from Microsoft Visio. A similar format can be created in Microsoft Project through its customization features. As the guide in Figure 9 shows, there are seven pieces of information for each task. Like the Gantt chart, each task has a name and duration. The duration is typically expressed in days. The early start and early finish blocks are filled out first starting with the first task and continuing sequentially until the last task is completed.
The first task starts at day zero (0) which is the early start. The early finish can be found by adding the task duration to the early start. The early start for the next task is the same as the early finish for the previous task. If one task feeds into multiple tasks, the early start of the multiple tasks will be the same as is the case with tasks B and C in Figure 9. When two or more tasks feed into a single task, the early start of the single task will be the latest early finish for the preceding tasks. For example, tasks B and C feed into task D in Figure 9. Task B has an early finish of 5 and task C has an early finish of 12. The latest early finish is 12, so that is the early start of task D.
Once the early start and early finish blocks of all the tasks are completed, then the late start and late finish blocks can be determined starting with the final task. The late finish block is determined first. For the final task, the late finish and the early finish are the same. The late start can be found by subtracting the task duration from the late finish. For example, in Figure 9 the late finish for task D is 15 because it is the same as the early finish for the final task. The late start is calculated by subtracting the task duration (3) from the late finish (15) which yields 12. The slack is determined by subtracting the early start from the late start (or the early finish from the late finish).
For items on the critical path, the early days and late days will be the same and there will be no slack time.
While working backwards, the late start of a task is the same as the late finish of the previous task or tasks. When there is more than one task with different late starts which is derived from a single task, the earliest late start is used as the late finish of the previous task. As an example, tasks B and C in Figure 9 have late starts of 8 and 1 days respectively. The late finish of task A will be the lower of these two numbers (1).
When the PERT chart is completed, the critical path is the tasks without slack time. In the case of Figure 9 the critical path is tasks A, C and D. If any one of the tasks would take longer than the estimated time, the end date of the project would be affected. Task B is not on the critical path. This task could take longer (up to 7 days longer) and it would not affect the end date of the project.
From a project management point of view, the tasks on the critical path need to be tracked more closely than the tasks that are not on the critical path. This allows the project manager to focus his or her time with the areas that are most likely to affect the schedule.
The PERT chart is more complicated than the Gantt chart but does provide the project manager with more information. The PERT chart is typically not used in presentations or in meetings with stakeholders due to its complexity.
Fortunately, project management tools such as Microsoft Project can be used to show both the Gantt and the PERT view so a project manager would not necessarily need to make two separate diagrams.