Using PERT analysis
By Ron Holohan
Improving the accuracy of schedule estimates
There are a few ways that you can increase the accuracy of your estimates for task duration in your project schedule.
One way is to use your own past experience and the past experience of others who have done something similar in previous projects. For example, you could ask questions related to past projects, such as:
- How long did the task take?
- What were some of the challenges you encountered?
- What would you do differently if you had to do it over again?
You should make note of any differences between the new task and similar tasks done in the past and take into account those differences when estimating a task’s duration. You should also consider that:
- Durations can depend on the experience of the resource performing the task. A highly experienced resource can sometimes complete certain tasks more quickly than a less experienced resource.
- Estimates should be revisited when work starts. You will know more about the tasks at that point, and your original assumptions may need to be changed.
You can also derive your estimates by comparing optimistic, pessimistic, and expected durations of a task. This is sometimes called a Program, Evaluation, and Review Technique (PERT) analysis, which many scheduling programs, such as Microsoft Project, support. By using a weighted average, a more predictable outcome can be determined. Usual PERT weights default to weighing the optimistic duration by 1x, the expected duration by 4x, and the pessimistiic duration by 1x.
Using PERT in MS Project
To use PERT analysis in Microsoft Project,
- On the View menu, point to Toolbars, and then click [PERT Analysis].
- On the PERT Analysis toolbar, click the [PERT Entry Sheet] button.
- For each task, enter the optimistic, expected, and pessimistic durations in the Optimistic Dur., Expected Dur., and Pessimistic Dur. fields, respectively. Note that if a task’s duration is not expected to vary, enter the expected duration in all three fields.
- Click the [Calculate PERT] button to calculate the estimated durations. Microsoft Office Project 2003 estimates a single project duration based on a weighted average of the three duration values for each task.
- To view the optimistic, expected, and pessimistic durations, on the PERT Analysis toolbar, click [Optimistic Gantt] button, [Expected Gantt] button, or [Pessimistic Gantt] button.
- To view the end dates of the three resulting schedules, on the Tools menu, click [Options].
- Click the View tab, and then select the Show project summary task box.
Let’s imagine that we are a project manager of a simple project made up of 3 interdependent tasks, each expecting to be of approximately 1 month in duration, as shown in Figure 1.
We want to better predict the project completion date milestone by estimating the optimistic, expected, and pessimistic durations for each task, as shown in Figure 2, by selecting the PERT Entry Sheet button from the PERT toolbar.
Once the estimated durations are entered, we can view the optimistic, expected, and pessimistic schedules by selecting the appropriate [Optimistic Gantt], [Expected Gantt], and [Pessimistic Gantt] buttons from the PERT Analysis toolbar.
The new weighted schedule can be created by selecting the [Calculate PERT] button from the toolbar. The new weighted Gantt chart (Figure 3) can then be seen by going to the View menu, and selecting Gantt Chart.
This schedule now shows the completion date to be June 15th based on weighted PERT Analysis.
Ron Holohan is currently a Program Manager for a major company in the Chicago, Illinois area, managing a group of very talented project managers and in the process learning from them. Rob Holohan’s blog can be found at pm411.org