PERT planning involves the following steps that are described below.
1. Identify the specific activities and milestones. The activities are the tasks required to complete a project. The milestones are the events marking the beginning and the end of one or more activities. It is helpful to list the tasks in a table that in later steps can be expanded to include information on sequence and duration.
Determine the proper sequence of the activities. This step may be
evident for some tasks. the exact order in which
Other tasks may require they must be performed.
3. Construct a network diagram. Using the activity sequence information, a network diagram can be drawn showing the sequence of the serial and parallel activities. Each activity represents a node in the network, and the arrows represent the relation between activities. Software packages simplify this step by automatically converting tabular activity information into a network diagram.
Estimate the time required for each activity.
Weeks are a commonly
used unit of time for activity completion, but any consistent unit of time can be used. A distinguishing feature of PERT is its ability to deal with uncertainty in activity completion time. For each activity, the model usually includes three
Optimistic time – generally the shortest time in which the activity can be completed. It is common practice to specify optimistic time to be three standards deviations from the mean so that there is a approximately a 1% chance that the activity will be completed within the optimistic time.
Most likely time – the completion time having the highest probability. Note that this time is different from the expected time.
Pessimistic time – the longest time that an activity might require. Three standard deviations from the mean is commonly used for the pessimistic time.
PERT assumes a beta probability distribution for the time estimates. For a beta distribution, the expected time for each activity can be approximated using the following weighted average:
Expected time = ( Optimistic + 4 x Most likely + Pessimistic ) / 6
This expected time may be displayed on the network diagram.
To calculate the variance for each activity completion time, if three
standard deviation times were selected for the optimistic and pessimistic times, then there are six standard deviations between them, so the variance is given by:
[ ( Pessimistic - Optimistic ) / 6 ]