Project 2007: Estimating Task Durations Using the PERT Formula
written by: Linda Richter
• edited by: Michele McDonough
• updated: 8/6/2011
Coming up with accurate task duration estimates is an age-old project management dilemma. One of these tools is built into Office Project 2007—Program Evaluation and Review Technique (PERT) Analysis. Here you will learn to use the function that calculates this and also how to understand the formula.
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How Long Will It Take?
PERT Analysis uses a weighted average of three duration estimates to calculate an estimated duration for a task. This is a great tool when you don't have a specific duration but do have duration estimates from reliable sources. Who are reliable sources? A reliable source is anyone (or anything) with experience working with this type of task, including, but not limited to, skilled project managers, the people who will be performing the work, and even the data logs of a resource like a machine (to find out how fast the equipment can get the job done).
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Manually Calculate Task Duration
Before you begin using Microsoft Project's PERT analysis tool, you must become familiar with the Estimated Duration formula to manually calculate a task's duration the same way Office Project 2007 would. To properly estimate task durations (either manually or using Office Project 2007), you'll need access to three numbers:
Most Pessimistic Time
Most Optimistic Time
With these three numbers, you and Office Project 2007 can create accurate task estimate durations. Each of these numbers holds a weighted average in Office Project 2007.
The Most Optimistic and Most Pessimistic times each hold a weight of one (or 16.66 percent each).
The Expected Time holds a weight of four (or 66.67 percent).
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Is Manual Calculation Necessary?
You might be wondering why you need to learn how to manually calculate a task's duration manually. How can it be important when you have Office Project 2007 to do it for you? But it really is something you need to know. You should be able to tell by looking at the results of the PERT analysis if the duration is approximately where it should be. You can know this only if you know how to calculate the task duration manually.
After you get the hang of manually calculating task duration estimates, you're ready to move to using Microsoft Office Project's PERT Analysis feature. The calculation you worked with when manually calculating estimated durations is the same calculation that Office Project uses. This means that you'll still need access to three numbers: Expected Time, Pessimistic Time, and Optimistic Time.
To begin, you'll need to display the PERT Analysis toolbar. You do this by clicking View, choosing Toolbars, and choosing PERT Analysis.
After you display the PERT Analysis toolbar, you can use the Pert Entry Form button to calculate estimated task durations based on the most optimistic, most pessimistic, and expected durations you enter. To calculate an estimated task duration using the PERT Analysis toolbar, follow these steps:
On the PERT Analysis toolbar, click the PERT Entry Form button (it's the fifth one from the right).
In the PERT Entry dialog box, type a number for the Optimistic, Expected, and Pessimistic durations.
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A Word About Project 2010
If you have been looking for information about using the PERT tool in Project 2010, you won't find much anywhere. Many Project 2010 resources tell you to go directly to the Network Diagram view, but I find I have to type a lot of tedious information into the network diagram shapes. What I like better is to create a list of tasks while in the Gantt chart view, and then drag the task bars around in the Gantt chart until I have things in the order I want. The estimated time for your projects will often become clear as you consider project logistics, and the dates will be filled in automatically.
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PERT vs CPM
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. Part 2 of this article tells you exactly how to do that; but be mindful you must know the default numbers if you are taking the certification examination.