There are two types of estimated duration: deterministic and probabilistic. When the duration for a task isn’t likely to dry, the duration is deterministic. It’s easy to plan the duration for routine deterministic tasks. You can enter a duration based on the duration the last time the task was completed. Projects, however, aren’t largely populated with routine tasks; the nature of projects is non-roundness the new unexplored edge of an organization’s operations. Non-routine tasks that are subject to variation are probabilistic.
For example, converting from one accounting software system to another is a project that Involves installation, testing, turnover, and evaluation. Although a project may include an accounting component, managing accounts payable or payroll isn’t a project- it’s an organizational process assigned to a department.
If your organization swaps accounting packages every three years just for fun, you can examine project documentation from the last conversion for guidance on duration.
But the last conversion can only serve as a guide. The next conversion-your project will include different software, different personnel, new vendors, and other variations too numerous to name.
PERT,the Performance Evaluation and Review Technique, was developed In the early 1960 s by the U.S. Navy; Lockheed; and the , Allen & Hamilton consulting firm to manage the Polaris missile project.
This was a probabilistic project from wall to wall: More than three thousand contractors completed hundreds of thousands of work package many of which were new and unique. PERT has been credited with bringing the Polaris missile project in two years ahead of the original estimates. PERT is a statistical method to estimate duration.
At the core of PERT is an understanding that there are some things you can’t know with precision. If an activity is unique to your project, it makes more sense to think about a range of duration (three to five days) rather than a precise duration (exactly four days).
To Implement a PERT analysis, you develop not one, but three task duration: optimistic (best case), pessimistic (worst case), and expected.
The three duration are weighted and averaged to determine the task’s duration. The default PERT settings give the expected duration four times the weight of either the pessimistic or optimistic duration: PERT duration = (optimistic + 4 * expected + pessimistic Project 2000 provides tremendous support for PERT.After you enter the three duration for a task, Project calculates the PERT duration.
You can change the weights used in PERT calculations. And, unlike many other project-management software packages, Project tracks all three duration for each task, so you can view the optimistic, pessimistic, or expected duration at any point in the project.
Developing PERT Estimates
The PERT estimates arc often casually described as worst-case, best-case, and expected . (or planned) estimates. “Worst·case” and “best-case” aren’t really accurate descriptions, particularly if you have a vivid imagination. The absolute worst-case scenario imaginable has hundreds of hours poured into a twenty-hour task as resources are run over by streetcars, killed in hurricanes, or engaged in acts of sabotage. The best-case scenario includes hiring geniuses who reinvent processes on the fly, shaving fifty percent off of each task duration.
PERT isn’t about the worst or best, it’s about the likelihood of actual duration within three standard deviations of the mean. You don’t need to brush off your probability and statistics textbook to work with PERT.Here are working definitions for each of the three estimates: .
• Pessimistic: The task will require this number of hours or fewer 95% of the time (19 out of 20 times).
• Optimistic: The task will require this number of hours or more 95% of the time (19 out of 20 times).
• Expected: The most probable amount of hours the task will require.
With PERT,you can treat one occurrence out of 20 as a fluke (when resources are run over by streetcars, etc.), so you don’t have to consider the truly outrageous when you develop your estimates.
There are several good ways to develop estimates, and one truly bad method, which we’ll dispense with first.
Don’t develop a single expected estimate, and then add and subtract the same percentage for each task to create pessimistic and optimistic duration. (We’ve seen this happen In .organizations that require a PERT analysis as part of a project proposal.) Here are some methods that are useful:
Historical data For a task that’s been completed and documented once or twice before, collect the actual duration and project duration’s. Use the lowest value as the optimistic, the highest as the pessimistic, and the average of all actual values as the expected duration.
Best guess by experts Find people who’ve completed similar tasks, and get their best guesses for the three duration for your task.
Best guess by experts versus Ask people with expertise in the task area for their estimate of actual time. Use the most frequently guessed value as the expected duration. Use the lowest value as the optimistic, and the highest as the pessimistic duration.
You don’t have to use PERT to calculate every task in your project. If a task Is deterministic, you can enter the same value for all three estimates. To enter PERT estimates, open the PERT Analysis Toolbar and do the following:
1. Choose View Toolbar PERT Analysis.
2. On the toolbar, click PERT Entry Sheet to open the Entry Sheet.
3. Enter the optimistic, expected, and pessimistic duration for each task that you want to calculate a duration for with PERT. For deterministic tasks, enter the same duration in all three fields.
Troubleshooting the PERT Analysis Toolbar
The PERT Analysis toolbar is a COM Add-In that should .already be enabled. If the PERT Analysis toolbar does not appear on the toolbar list,follow these steps to make it available:
1. From the View menu, click Toolbar is then click Customize.
2. dick the Commands tab in the Customize dialog box Select the Tools category.
4. Select COMAdd-Ins from the Commands section and drag it to a toolbar.
S. dick the Close button to dose the Customize dialog box.
6. dick the COMAdd-ins button that you placed on a toolbar.
7. In the COMAdd-Ins dialog box, dick the Add button. L Switch to the folder where Winproj.exe (the Project program) resides.
9. locate a file named Pertan 1 . dl1. Select the file and clickOK.
10. ClickOKagain to close the Com Add-Ins dialog box.
11. Choose View> Toolbars > Pert Analysis toolbar to activate.
If you want to enter estimates for one task, click the PERT Entry form button on the PERT Analysis toolbar to open the PERT Entry Form.