How to Create a Project Evaluation and Review Technique and Critical Path Method (PERT-CPM) Network Chart
A Project Evaluation and Review Technique and Critical Path Method (PERT-CPM) Network Chart is a powerful tool in project planning. The PERT-CPM Network Chart indicates the flow of the steps necessary to undertake a project as well as the schedule of each of the steps. The chart also provides an overview on which of the steps should be taken with direct care in order to avoid any delays on the project.
To make a PERT-CPM Network chart, the following steps should be followed:
Are you familiar with network charts?
1. Create the head event.
A head event is the starting point of your task. Though you cannot see the head event on your task list, the head event is an important component that denotes both your initial setting and the results of your tasks. You can use countless of head events (depending on the number of tasks you have) but you can only have one primary head event – the first starting node that denotes the first task on your task list – on your network chart.
The head event is drawn similar to a circle labeled with a number as shown in the figure.
2. Draw an arrow to denote a task.
After creating the head event, draw an arrow pointing away from it (as shown in the image above). The arrow indicates the task that will be conducted, and only represents one and only one task. Corresponding events that require more than one task should be included in the dummy task (which will be discussed in the next step). The arrow is also labeled with the number of days required for the task to complete.
3. Create the tail event.
The tail event is the result of a task and will be the next head event in the next task. Tail events are drawn similar to the head event (i.e. a circle with a number label) and are not seen on the task list directly. Tail events are located at the head tip of the arrow (as shown in the image above).
You can create countless of tail events but, similar to head tasks, you can only create one primary tail event which would correspond to the end of the network chart.
4. If you have events that require two tasks, create a dummy task.
As the basic rule of network charts, no head and tail events can contain two or more of the same task. This means that for each event in the chart, only one arrow should be seen away from it and into another event (as seen in the illustration above). Still, there are times when we need to have more than one task for one event; which brings back the question, “How can we avoid creating two or more tasks for a single event?”
Dummy task then comes into the equation. A dummy task is any task with zero duration. It is drawn with a dashed arrow and labeled with zero to indicate that it requires no time duration for it to complete. Dummy tasks are especially helpful in order to avoid confusion on which of the task to undertake and create a meaningful flow of events.
5. Connect all the tasks.
Since you’ve known the basics of creating a network chart from the previous steps, it’s time to connect all the events. Continue making head events and tails events and connecting them with the task arrows until you arrive at the primary tail event. If you have any trouble placing the events, try to make a rough draft of the network chart. Just make sure that no task arrows overlap with each other.
6. Determine the Earliest Starting Time (EST) of each task.
The EST indicates the time you could start each task without any delay. The primary head event (i.e. the circle with the label one) has the EST labeled with zero (or t=0) and should be the starting point of the computation of the EST. To compute the EST of the corresponding events, simply add the EST of the previous event and the duration of the task (the labeled on the arrow such as A=3). For instance, if the previous event has the EST of 12 and the duration indicated on the task arrow is B=14, then the EST of the next event should be the sum of 12 and 14 which is 26. The sum of the EST and the duration of the task is referred to as the Earliest Completion Time (ECT).
In cases where there is more than one arrow pointing to the same tail event, the highest ECT among the tasks should be used as the EST of the tail event. For the case of dummy tasks, EST is simply copied from the previous event since time duration of the task is equal to zero.
ESTs of all events are drawn in the network chart as a blue or green square with the time label.
7. Determine the Latest Completion Time (LCT) of each task.
The LCT is the most delayed period of your project. LCT is computed quite similar to EST but in the backward direction. In other words, the starting point for all the computations starts at the primary tail event. The EST of the primary tail event will serve as the starting LCT. To compute for the LCT of the other events, simply deduct the LCT of the tail event and the duration of the task (as indicated by the label on the arrow). For example, if the LCT of the tail event is 24 and the duration indicated on the task is G=12, then the LCT of the head event will be the difference of 24 and 12 which is 12. The difference is also considered as the Latest Starting Time (LST) of the head event.
All of the events on the network chart must have an LCT. LCT is commonly drawn as a red triangle with a number label corresponding to the LCT of an event.
8. Identify the Critical Path.
The critical path is a set of all tasks and events with the EST=LST and ECT=LCT. Identifying it on the network chart is easy; simply look at the square and triangle of every event and see if the labels in it are the same. The critical path is an important factor in the project since it any delays on the tasks within the critical path (i.e. ended or started late) would mean the delay of the entire project.
Job Well Done
And there you have it. You just created your very own PERT-CPM network chart for your project. Though it was not entirely easy (especially for the computations and the dummy tasks), you still did a great job of creating it. And now you’ve made the first step on realizing your project, its time to work on it by following the network chart you’ve made as the guide. Have fun with your project!
Calvo, F. S. (n.d.). Research 2 - Knowledge, Integration, Application, and Extension. Cebu, Philippines.