Project Evaluation and Review Technique (PERT) is a network-based technique for planning and scheduling interrelated tasks in a project. Project managers use this methodology to optimize time and resources in large and complex projects. Most popular project related software such as Microsoft Project supports this methodology.
A PERT chart depicts the various events of a project, the duration of events, and dependency among various events. This captures the precedence or parallel relationships among the various events and help identify the possibility of performing multiple tasks simultaneously, to complete the project in the quickest possible time.
Events and Milestones
The first step in project planning with PERT is to identify project events and milestones. Any project consists of various activities broken down into specific units of tasks known as events. Milestones are events that mark the beginning and end of an activity.
Nodes depict each event, and circles or bubbles depict milestones. A line connects the various events in an activity. The information contained in the activity line includes name of the activity, the duration of the event, the number of people assigned for the event, and other identifiers.
The steps to list out events and milestones include:
- numbering events sequentially in 10s (10, 20, 30, etc.), to allow later insertion of additional events
- selecting specific events as milestones
Determining Sequence of Activity
The second step is to determine the sequence of activities. The PERT chart starts with an initiation node or event. The end event of the activity leads to another event or to slack time. The events branch out to various activities and then come together at the end to the completion node.
The nodes and connecting lines makes explicit the dependencies between events. The flow of nodes in a logical sequence reveals
- activities that cannot begin until the completion of its immediately preceding event, indicating serial activities
- activities that can run simultaneously, indicating parallel activities
Finalization of the activity sequence information usually requires several drafts. Software packages such as Microsoft Excel simplifies this step by automatically converting tabular activity information into the network diagram. Determining the sequence of activity for large and complex projects nevertheless extend to weeks.
Estimating Activity Times
The third step is to determine activity time. PERT incorporates three time estimates for the completion of each activity: optimistic time, most likely time, and pessimistic time. Optimistic time is the best-case scenario, or the time required if all activities run optimally without any hindrance. Most likely time is the normal time or standard time required, taking into account the real life situations and limitations. Pessimistic time is the worst-case scenario, factoring in all things that may go wrong.
The formula to calculate estimated time for the completion of an activity is Expected time = ( Optimistic + 4 x Most likely + Pessimistic ) / 6, based on beta probability distribution. The most common unit of time used is weeks.
Determining the Critical Path
Critical Path is the sequence of serial activities in the project. The time taken for completion of all serial activities in the project is usually the time required to complete the project, for activities outside this serial path run simultaneously. Speeding up or slowing activities outside this path does not change the total project time, provided such activities do not exceed their slack time, but speeding up or slowing critical path activities directly impacts project completion time.
Project planning with PERT can help plot this crucial path and fix the expected project completion time. Identification of this path help project managers devise ways to expedite or give priority to activities on this path over others. Such estimates are however purely subjective.
- Chinneck, John, W. Practical Optimization: A Gentle Introduction. Chapter 11: PERT for Project Planning and Scheduling
- Project Management Institute (2003). A Guide To The Project Management Body Of Knowledge (3rd ed.). Project Management Institute. ISBN 1-930699-45-X.
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