How to build a PERT diagram
A PERT diagram is a project planning and management tool. Its graphic form represents project illustrating connected nodes and arrows to specify the main steps of a project, and follow up the project evolution.
Origin of the PERT method
The Program Evaluation and Review Technique (PERT) method is a project management tool used to schedule and design the tasks of a project. The resulting PERT diagram allows a direct visualization of all the tasks, their interconnections and dependencies, and the time required to realize them.
This method was developed by the US Navy in 1959 to follow and manage the development progress of the Polaris Fleet Ballistic Missile Program.
Definitions of the PERT chart elements
A PERT diagram is constituted of tasks represented by arrows, and steps represented by circles. A step is the beginning and the end of a task.
Representation of a step in a PERT diagram.
The circle is divided in 3 parts: the upper part contains the number of the step, the lower left part contains the earliest start time of the step, and the lower right part contains the latest start time of the step.
Representation of a task in a PERT diagram. Above the arrow is indicated the name of the task, and below is mentioned the duration of the task.
How to build a PERT diagram
The first step for building a PERT diagram consists in listing all the tasks of your project. Then, identify inter-dependencies between the tasks, and estimate the duration to complete them. These data can be gathered in a table. An example is given in the table below.
You can then draw your PERT diagram, starting by the first step (n°1), and link it to the direct following tasks (A & B) materialized by arrows. Since those tasks do not require previous tasks, they can start simultaneously. Below tasks A & B is indicated the time required to perform them, i.e., 3 and 4 respectively. Once accomplished, these tasks lead to steps 2 and 3. Task C follows task A (as indicated in the table), and task D follows task B. Since task E requires both task C and D to be terminated, C and D lead to the same task (n°4). Task E being the last task of the project, it leads to the final step (n°5).
Next, you should indicate on the diagram the time dependencies. You will start with indicating the earliest start dates, i.e., the lower left part of the circles. The first step starts at 0. Then you will add the times required for each task. For step A, which requires 3 hours to be accomplished, the earliest start date is 3 hours. For step B, the earliest start date is 4 hours. When several tasks converge towards the same step (which is the case for C and D), retain as the earliest date the greatest number of days of the different possibilities. So here you should add the earliest time at step 2 and the time to perform task C i.e., 3h + 5h = 8h. Finally, add the time to perform task E which is 2h, to 8h + 2h = 10h.
The final step consists in indicating on the diagram the latest start dates. Start from the end of the diagram, indicate the latest date equal to the earliest date, here 10 hours. Then follow the reverse path by subtracting this time from the latest date of the considered step, i.e., the duration of the task that precedes it to find the latest date of the step positioned upstream. For task 4, this corresponds to 10h – 2h = 8h.
Advantages of a PERT diagram
This diagram allows to highlight several points which are crucial in project management:
The margins correspond to the delay that can take place in completing a task without affecting the project. In our example, we can see that task B can be completed in 4 to 5 min without impacting the rest of the project.
The critical path is the one with no margin, meaning that no task should be delayed on this path to avoid delaying the whole project.
Although easier to understand, a Gantt diagram does not reflect inter-dependencies of tasks and the critical path. A PERT diagram enables the project manager to quickly identify the risks of a project.
Are you interested in project management tools? Stay tuned, we will soon publish a post about the Gantt chart. Subscribe to NETO Innovation website to receive these posts directly in your inbox.
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Malcolm, D. G., J. H. Roseboom, C. E. Clark, W. Fazar. "Application of a Technique for Research and Development Program Evaluation," Operations Research, Vol. 7, No. 5, September–October 1959, pp. 646–669