You have many ways to gather information to estimate durations. However, most of these methods don't enable you the flexibility of analyzing bestcase and worstcase scenarios. A Program, Evaluation, and Review Technique (PERT) analysis gives you this flexibility.
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About the PERT analysis
You can perform a basic PERT analysis to estimate a task's duration. After you specify the optimistic, pessimistic, and expected durations of the tasks in your schedule, Microsoft Office Project 2007 calculates a weighted average of the three durations. You can also use these durations separately to determine a shortest, longest, and most likely project end date.
After you assign duration amounts to all three types, you can look at your schedule based solely on the optimistic, pessimistic, or expected duration. You can also have Project 2007 calculate a single duration based on the weights for each of the types and the duration values that you entered.
You can perform a PERT analysis in two ways:

Use the default weights for duration estimates if you think that the expected duration estimate is more likely than either the optimistic or pessimistic estimate, and you think the latter two estimates are equally likely.

Change the way that Project 2007 weights duration estimates if you think that your optimistic, expected, and pessimistic durations have probabilities of occurring that are different from the default probabilities of 1 out of 6, 4 out of 6, and 1 out of 6, respectively.
For example, one of the tasks in your project has an optimistic duration of 3 months, an expected duration of 4 months, and a pessimistic duration of 6 months. Using the default weights, the optimistic duration is counted once, the expected duration is counted four times, and the pessimistic duration is counted once, as shown in the following table:
Optimistic 
Expected 
Pessimistic 

3 months 
+ 
4 months + 4 months + 4 months + 4 months 
+ 
6 months 
The sum of this calculation, 25, is divided by 6 and rounded to two decimal places to reach the weighted average of 4.17 months. This average is inserted in the Duration column for your task.
Note: The PERT analysis provided by Project 2007 is focused on the task level. Summarylevel and overall project data is not available for analysis by using Project 2007.
Perform a PERT analysis by using the default weights for duration estimates

On the View menu, point to Toolbars, and then click PERT Analysis.

On the PERT Analysis toolbar, click PERT Entry Sheet .

For each task, enter the optimistic, expected, and pessimistic durations in the Optimistic Dur., Expected Dur., and Pessimistic Dur. fields, respectively.
If a task's duration is not expected to vary, enter the expected duration in all three fields.

Click Calculate PERT , and then click Yes in the dialog box that appears.
Project 2007 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 , Expected Gantt , or Pessimistic Gantt .
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 check box.
Perform a PERT analysis by changing the way Project 2007 weights duration estimates

On the View menu, point to Toolbars, and then click PERT Analysis.

On the PERT Analysis toolbar, click Set PERT weights .

Change the number in at least two of the following fields — Optimistic, Expected, and Pessimistic — so that the sum of all three numbers equals 6.

On the PERT Analysis toolbar, click PERT Entry Sheet .

For each task, enter the optimistic, expected, and pessimistic durations in the Optimistic Dur., Expected Dur., and Pessimistic Dur. fields, respectively.
If there is a task duration that you do not want to have calculated, enter the same duration in all three fields.

Click Calculate PERT , and then click Yes in the dialog box that appears.
Project 2007 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 , Expected Gantt , or Pessimistic Gantt .
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 check box.