A PERT chart, sometimes called a PERT diagram, is a project management tool used to schedule, organize and coordinate tasks within a project. It provides a graphical representation of a project's timeline that allows project managers to break down each individual task in the project for analysis.
The PERT chart template uses nodes -- drawn as rectangles or circles -- to represent events and milestones throughout the project. The nodes are connected by vectors -- drawn as lines -- which represent the various tasks that need to be completed.
PERT charts provide project managers with an estimation of the minimum amount of time needed to complete a project. Managers can also analyze the work breakdown and task connections as well as assess the risk associated with the project. The breakdown structure makes it easy to organize a complex project with a variety of moving parts by visualizing the dependencies between each step of the process.
PERT stands for Program Evaluation Review Technique, a methodology developed by the U.S. Navy in the 1950s to manage the Polaris submarine missile program. A similar methodology, the Critical Path Method (CPM) was developed for project management in the private sector at about the same time.
Nodes are visual representations of milestones or events within the project. They are drawn as either numbered boxes or numbered circles.
Arrows are visual representations of the tasks that occur throughout the project. The direction of the arrow indicates the sequence of the task. Diverging arrows show that various tasks can be completed at the same time.
Fast tracking is when tasks and activities are performed simultaneously.
A PERT event is the point at which one or more tasks are started or completed.
A predecessor event occurs immediately before some events. A successor event naturally occurs after events.
Slack is the amount of time a single task can be delayed without harming other tasks or the project as a whole.
The critical path is the longest -- or most time consuming -- path from the start to the completion of an event or task.
Critical path activity refers to a task that does not experience slack.
Crashing critical path is when the completion time of a task is shortened.
Lag time refers to the earliest point at which a task can follow another.
Lead time is the amount of time it should take to complete a task without impacting the following activities.
Expected time is the best estimation of how long a task will take to complete, taking into consideration any problems or obstacles that might arise.
Optimistic time refers to the minimum amount of time it will take to complete a task.
Pessimistic time is the maximum amount of time it will take to finish a task.
Most likely time is the best guess of how long a task will take, assuming no problems arise.
How a PERT chart works
A PERT chart presents a graphic illustration of a project as a network diagram consisting of numbered nodes linked by labeled vectors. The direction of the arrows on the lines indicates the sequence of tasks.
When creating a PERT chart, project managers should follow the below steps:
- Define all activities involved in the project.
- Consider any dependencies between tasks.
- Draw nodes and arrows based on the information gathered in the first two steps.
- Identify the completion time for each task.
Once the PERT chart is drawn, project managers can use it to develop a realistic timeframe for the project. This can be done by finding the longest path, based on the estimations entered. This path should include the tasks that take the longest to complete. By adding together the time it takes to complete each task, an estimation will be provided for how long the entire project will take. Once this is done, the PERT chart can then be used to adjust the times if circumstances change or to ensure the project is finished by its designated deadline.
A diagram and example of a PERT chart.
In the diagram, for example, the tasks between nodes 1, 2, 4, 8 and 10 must be completed in sequence. These are called dependent or serial tasks. The tasks between nodes 1 and 2, and nodes 1 and 3 are not dependent on the completion of one to start the other and can be undertaken simultaneously. These tasks are called parallel or concurrent tasks. Tasks that must be completed in sequence but that don't require resources or completion time are considered to have event dependency. These are represented by dotted lines with arrows and are called dummy activities. For example, the dashed arrow linking nodes 6 and 9 indicates that the system files must be converted before the user test can take place, but that the resources and time required to prepare for the user test (writing the user manual and user training) are on another path. Numbers on the opposite sides of the vectors indicate the time allotted for the task.
When to use a PERT chart
PERT charts should be used when a project manager needs to:
- Determine the project's critical path in order to guarantee all deadlines are met.
- Display the various interdependencies of tasks.
- Estimate the amount of time needed to complete the project.
- Prepare for more complex and larger projects.
Project managers find best results when they use a PERT chart at the beginning of the project. Doing so helps ensure the project is accurately scoped and can help avoid any potential bottlenecks in the process.
Advantages of PERT charts
The PERT technique provides managers with the ability to evaluate the time and resources necessary to a project by tracking required assets at each stage of the process as well as throughout the course of the project. Furthermore, PERT charts are useful in what-if analyses, helping companies understand all possible workflows and choose the most efficient and beneficial path.
The analysis of the PERT chart includes data from various departments within an organization. Combining all of the information helps identify each responsible team within the company while facilitating an environment where each department takes responsibility for their work. The process of creating a PERT chart also improves communication and enables an organization to invest energy in projects that will enhance its strategic positioning.
Other advantages include: making unclear deadlines more predictable; clarifying dependencies between tasks; and establishing a clear order for completing the tasks.
Disadvantages of PERT charts
Disadvantages of the PERT chart include:
- A strict focus on deadlines may not allow managers to see the full financial positioning of the project.
- PERT charts lack the flexibility needed to adapt to small changes that occur when confronted with a roadblock.
- If any calculations are inaccurate in the creation of the chart, then delays could occur, causing bottlenecks and negatively impacting the final delivery date.
- PERT charts are subjective; their success depends on the experience of the project manager. Consequently, some charts may include unreliable data or unrealistic expectations for the cost and time frame of the project.
- Creating a PERT chart is labor intensive, requiring additional time and resources. Also, in order for the chart to remain valuable, it must be consistently reviewed and maintained.
PERT chart vs. Gantt chart
The PERT chart is sometimes preferred over the Gantt chart, another popular visual tool for project managers, because it clearly illustrates task dependencies. On the other hand, the PERT chart can be much more difficult to interpret, especially on complex projects. Frequently, project managers use both techniques.
Both the PERT and the Gantt charts help simplify the project management process through a breakdown structure that divides the project into smaller tasks and identifies any constraints, thus allowing managers to increase efficiencies and view the project as a whole. Another similarity is the use of both charts to identify dependencies and improve time management.
However, while PERT charts are best used before a project begins -- to plan and estimate how long each task will take -- Gantt charts are best utilized while a project is running. Throughout the project, managers use Gantt charts to schedule tasks by date and visualize the amount of completed work. In a Gantt chart, each task is represented by a long bar that connects the starting point of the task to its end date.