The Program Evaluation and Review Technique (PERT) is considered a project management classic. Besides being one of the original scheduling techniques: PERT was the first significant project-oriented risk analysis tool. PERT'S objectives included managing schedule risk by establishing the shortest development schedule, monitoring project progress, and funding or applying necessary resources to maintain the schedule. Despite its age (relative to other project risk techniques), PERT has worn the test of time well.
Technique Description
PERT is based on a set of mathematical equations known as Runge-Kutta.
The best- and worst-case scenarios are established and weighted against the most likely set of occurrences. PERT mean and standard deviations and the project's PERT duration and standard deviations are established for each task in a project network, which allows the project manager to evaluate the likelihood of achieving specific schedule targets based on the network and PERT durations.
When Applicable
PERT is particularly applicable when historical schedule data are limited.
In many projects, there is not sufficient information to ascertain precisely how long a given task might take; or sometimes team members are reticent about sharing planned duration for activities they have never performed.
By allowing or encouraging each team member to provide a best-case duration, a worst-case duration, and a most probable duration for each activity, team members have the opportunity to share information they might not otherwise have considered (in a single data-point estimate).
PERT was originally developed during the Polaris submarine program in the late 1950s.
Consequently, PERT is normally applied early in a project when uncertainty is high.
Inputs and Outputs
Inputs for PERT include the multiple duration data points for each activ- ity and the basic network of activities (Chapter 19). Gathering this infor- mation may require a significant level of effort, but it is normally tracked with the work packages in the project management software. Most mid- to high-range project management software packages incorporate PERT fields in their databases.
Outputs from PERT are mean durations for the project's critical path, as well as normal distribution curves to establish the likelihood of meeting various schedule targets. These outputs are normally more pessimistic than the duration derived from critical path method analysis because they take the best and worst cases into account (and worst-case scenarios tend to diverge further from the most likely duration than do best-case scenarios).
Thus, PERT duration reflects more risks inherent in the network and the project as a whole.
Major Steps in Applying the Technique
PERT is applied in two general phases, first at the task level and again at the project level.
At the task level, there are three steps that must be conducted for each task:
Gather the task duration information. As mentioned earlier, this will consist of establishing best-case, worst-case, and most likely durations for each task in the network. This information is normally extracted from individual team members performing the task.
Calculate the PERT mean and standard deviation for each task. This is frequently done by using computer tools, although it can be calculated manually. For the PERT mean, the following formula is applied:
O~timistic
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(4 X Most Likelvj+
Pessimistic 6To establish the PERT standard deviation, some of the same information is used:
Pessimistic
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Catalog the information. Storing the information for easy retrieval is important because PERT data at the task level have limited utility.
That may be helpful for establishing the basic duration of a task, but to apply the robust nature of the PERT process, the entire network must be considered.
A t the project level, there are three steps that must be conducted once PERT information is available for each task:
Establish the PERT critical path. The project manager must calculate the critical path based on PERT durations rather than the conventional, most probable durations. Because PERT durations frequently differ from their most likely counterparts, there is the distinct possibility that the PERT critical path will represent a different set of activities than the conventional critical path. The duration of this path becomes the PERT mean for the project.
Establish the standard deviation for the PERT critical path. This tends to be one of the more confusing steps in the process since it involves - -
calculation of the square root of the sum of the squares of the task- developed standard deviations. The process (once again, frequently performed by computers rather than people) is not as onerous as it might sound. First, square each of the individual task standard deviations. Then add those squares together. Finally, calculate the square root of their sum. The formula looks like this:
That number ~rovides the standard deviation for the PERT duration of the project as a whole. It is noteworthy that this number is significantly smaller than the sum of the standard deviations for the project's PERT estimates. That is because it factors in the reality that not all activities will occur in their worst case on the same project. It also acknowledges that although some activities may be delayed, that will probably not be the case for the entire network.
Plot the PERT mean and standard deviation into a distribution. There are two fundamental approaches to assessing the distribution of activities under a PERT mean. The first is the classic approach to normal dis- tributions with a curve like the one in Figure 20.
In this scenario, the assumption is that there is a 68.26 percent chance that the duration of the project will occur within one standard deviation of the PERT mean. There is a 95.4 percent chance the
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