## Calculating Your Most Probable Duration

In the course of researching task duration information, you might get conflicting results. Maybe the team member who will carry out a major task says it will take 3 weeks. Perhaps an expert stakeholder says it should take 2 weeks. And maybe the industry standard states that the same task should take 4 weeks. These are large discrepancies and they're all coming from credible sources. How do you schedule a task with three possible durations?

Or maybe you have a single reliable duration or a duration range such as 2 weeks +/- 10 percent for all tasks in your task list, and you want your project plan to model a best-case scenario, a worst-case scenario, and an expected scenario for all durations. This way, you can learn the earliest possible project finish date, the latest possible date, and the most probable finish date.

To help resolve discrepancies or to model alternative scenarios, you can run a PERT analysis. A PERT (Program Evaluation and Review Technique) analysis uses a weighted average of optimistic, pessimistic, and expected durations to calculate task durations and therefore the is project schedule. This analysis can be an effective risk management tool. It can also help if j|

you're working out a project proposal or estimating time, cost, or resource requirements. °

Caution When you run a PERT analysis, the resulting calculated values in the Optimistic Duration, Expected Duration, and Pessimistic Duration fields will be stored in the custom fields Durationl, Duration2, and Duration3, respectively. In addition, the resulting optimis tic start and finish dates are stored in the custom fields Startl and Finishl. The expected start and finish dates are stored in the custom fields Start2 and Finish2. The pessimistic start and finish dates are stored in the custom fields Start3 and Finish3. Any values in any of these custom fields are overwritten by the results of the PERT analysis. This can be sig nificant if you were storing interim plan information in these fields.

PERT Entry Sheet

To set up a PERT analysis, follow these steps:

1 Click View, Toolbars, PERT Analysis.

2 On the PERT Analysis toolbar, click PERT Entry Sheet.

3 For each task, enter the optimistic, expected, and pessimistic durations in the appropriate fields (see Figure 5-5). You can also think of these fields as minimum, probable, and maximum durations for each task.

Calculate PERT

3"

 Task Name Duration Optimistic Dur. Expected Dur. Pessimistic Dur. 16 E Foundation 1 day? 0 days 0 days 0 days 17 Excavate for foundations 1 day? 2 days 3 days 4 days 18 Form basement walls 1 day? 11 days 13 days 18 days 19 Place concrete for foundations a basement walls 1 day? 10 days 12 days 16 days 20 Cure basement walls for 7 days 1 day? 5 days 7 days 10 days 21 Strip basement wall forms 1 day? 1 day 2 days 4 days 22 Waterproof J1 n s u I ate basement walls 1 day? 2 days 2 days 4 days 23 Perform foundation inspection 1 day? 1 day 1 day 3 days 24 Backfill foundation 1 day? 1 day 2 days 5 days 25 □ Framing 1 day? Odays Odays Odays 26 Install 1 st floor joists 1 day? 1 day 2 days 3 days 27 Lay 1 st floor decking 1 day? 2 days 2 days 4 days 28 Frame 1 st floor walls 1 day? 3 days 4 days 8 days 29 Frame 1st floor corners 1 day? 1 day 1 day 2 days

Figure 5-5. Use the PERT Entry Sheet to specify the optimistic, expected, and pessimistic durations for each task.

If you do not expect a duration for a particular task to vary at all, enter the same value in all three fields.

On the PERT Analysis toolbar, click Calculate PERT.

The estimated durations are calculated, and the results change the value in the Duration field (see Figure 5-6).

 Task Name Duration Optimistic Dur. Expected Dur. Pessimistic Dur. 16 E Foundation 13.5 days? 11 days 13 days 18 days 17 Excavate for foundations 3 days? 2 days 3 days 4 days 18 Form basement walls 13.5 days? 11 days 13 days 18 days 19 Place concrete for foundations & basement walls 12.33 days? 10 days 12 days 16 days 20 Cure basement walls for 7 days 7.17 days? 5 days 7 days 10 days 21 Strip basement wall forms 2.17 days? 1 day 2 days 4 days 22 Waterproof J1 nsu 1 ate basement walls 2.33 days? 2 days 2 days 4 days 23 Perform foundation inspection 1.33 days? 1 day 1 day 3 days 1 24 Backfill foundation 2.33 days? 1 day 2 days 5 days I 25 E Framing 4.5 days? 3 days 4 days 8 days g 26 Install 1 st floor joists 2 days? 1 day 2 days 3 days i|27 Lay 1 st floor decking 2.33 days? 2 days 2 days 4 days 1 28 Frame 1 st floor walls 4.5 days? 3 days 4 days 8 days Ij 29 Frame 1st floor corners 1.17 days? 1 day 1 day 2 days

Figure 5-6. The recalculated durations based on the PERT analysis replace the values in the Duration field for each task.