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.

For more information about interim plans, see "Saving Additional Baselines" on page 304. For more information about using custom fields, see "Customizing Fields" on page 788.

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.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

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Responses

  • frodo headstrong
    How do you calculate probable duration?
    3 months ago

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