# Use a PERT analysis to estimate task durations

You have many ways to gather information to estimate durations (duration: The total span of active working time that is required to complete a task. This is generally the amount of working time from the start to finish of a task, as defined by the project and resource calendar.). However, most of these methods don't enable you the flexibility of analyzing best-case and worst-case scenarios. A Program, Evaluation, and Review Technique (PERT) analysis (PERT analysis: PERT [Program, Evaluation, and Review Technique] analysis is a process by which you evaluate a probable outcome based on three scenarios: best-case, expected-case, and worst-case.) gives you this flexibility.

- About the PERT analysis
- Perform a PERT analysis by using the default weights for duration estimates
- Perform a PERT analysis by changing the way Project 2007 weights duration estimates

## About the PERT analysis

You can perform a basic PERT analysis to estimate a task's (task: An activity that has a beginning and an end. Project plans are made up of tasks.) duration. After you specify the optimistic (optimistic duration: The best-case possibility for the total span of active working time expected for a task, that is, the amount of time from the optimistic start to optimistic finish of a task.), pessimistic (pessimistic duration: The worst-case possibility for the total span of active working time expected for a task, that is, the amount of time from the pessimistic start to pessimistic finish of a task.), and expected (expected duration: The total span of active working time expected for a task, that is, the amount of time from the expected start to the expected finish of a task.) 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 (schedule: The timing and sequence of tasks within a project. A schedule consists mainly of tasks, task dependencies, durations, constraints, and time-oriented project information.) 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. Summary-level 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.