Project 2007: Applying Different PERT Weights

In Part 1 of this article, you learned exactly how to calculate a PERT analysis by hand. Knowing how and why it's calculated helps you to make better management decisions and also helps you to judge whether some part of your task list should be adjusted or otherwise managed.

You can use the PERT Entry Form to estimate and then enter estimated durations for your project's tasks. Those durations are then added to your project file under PERT Analysis fields. In fact, you can view those fields using one of the first three buttons on the PERT Analysis toolbar.

There are three options: Optimistic Gantt, Expected Gantt, and Pessimistic Gantt.

Optimistic Gantt

• This button displays the Gantt Chart along with the PA_Optimistic Case table. The Opt Duration fields of any task that uses the PERT Form calculation is displayed and the project's Gantt Chart is updated to reflect the most optimistic outcome.

Expected Gantt

• This button displays the Gantt Chart along with the Entry table. The Exp Duration fields of any task that uses the PERT Form calculation is displayed and the project's Gantt Chart is updated to reflect the expected outcome.

Pessimistic Gantt

• This button displays the Gantt Chart along with the PA_Pessimistic Case table. The Pes Duration fields of any task that uses the PERT Form calculation is displayed and the project's Gantt Chart is updated to reflect the most pessimistic outcome.
• Viewing the PA fields

You might need to move the Gantt Chart divider bar to the right to display the fields in the Gantt Chart table. When you click these PERT Analysis toolbar table buttons, you might not see an immediate change in the table data. If this happens, drag the divider bar to the right to display the newly added columns.

After you've entered PERT Analysis figures in the PERT Entry form, you can apply those calculation results to your Entry table by clicking the Calculate PERT button on the PERT Analysis toolbar. Be aware that if you choose to enter task durations in this way, Office Project 2007 will overwrite any durations and Start or Finish dates you have entered for tasks utilizing the durations from the PERT Analysis form. If you haven't read up on using the Pert Analysis toolbar, check out the first part of this article, called Project: Estimating Task Duration Using the PERT formula (Part 1 of 2).

When you enter your optimistic, expected, and pessimistic durations into the PERT Entry form, you can change how much weight you apply to each one. For example, maybe you want to apply more weight to the optimistic view--everything's looking really good for your project. You can change the duration weights, but keep in mind the three numbers must add up to 6. So, if you increase the optimistic option to 2, you must likewise decrease one of the other durations. Click on the icon that looks like a set of scales to access this function.

What if your expected task duration is 4 days, your pessimistic estimate is 6, and your optimistic estimate is 2. Project is calculating 4--times 4, because it gives it four times more weight to expected duration--and then it adds in one 6 and one 2, and divides it all by 6. So you have 16+2+6 = 24 divided by 6 is 4. Let's look at changing each estimate: Your optimistic view is weighted 2, (2+2); expected is weighted 3 (4+4+4); pessimistic is weighted 1 (6) for a total of 22 divided by 6 = 3.66 days. You're better off letting Project use its default weights, but I wanted you to see how it works.

By default, Office Project 2007 calculates based on the Critical Path Method (CPM), which forecasts the project's total duration by analyzing the least amount of task scheduling flexibility. In project management you can use the PERT Analysis method for estimating task durations and CPM to manage task importance by defining task relationships and constraints. This way, CPM and PERT can work hand-in-hand.

Using the Set PERT Weights button on the PERT Analysis toolbar, you can modify the default weights given to any of the formula variables. However, for the purposes of the certification exam, be sure you understand the default values and use those in any calculations you're asked to perform.

This adapation includes excerpts used with permission from Microsoft Press from the book, Managing Projects with Microsoft Office Project 2007 (Self-Paced Training Kit for Exam 70-632) and is augmented by the writer's experience with Project, specifically Project 2010.

Question mark - sxc.hu, tulp. Royalty-free license.

Screenshot is supplied by the writer.