Event in Network Diagram (PERT & CPM)

What is an event in Network Diagram Analysis:

The starting and ending points of an activity are called events or nodes in a network diagram. Activities represent the actual work performed in a project, whereas events simply indicate milestones in the project's progress.

An event is that particular moment of time when a specific part of a project is to be completed, whereas an activity is the actual performance of a task. Words like complete, start, issue, approves, taste, and so on are frequently used to describe events.

For example, "Foundation Completed," "Design Approved," "Materials Received," and "Machine Installed" are examples of events because they indicate the completion or occurrence of a specific stage in a project.

An event in a network diagram represents the beginning or completion of one or more activities. It is a specific point in time that marks the occurrence of an important stage in a project. Unlike an activity, an event does not involve any work; rather, it indicates that a particular task has started or has been completed.

Operations Research Notes

Network Analysis: Definition, Components, Applications, CPM and PERT

Activities can be included within events. As an example, a cultural festival is an event that includes a variety of activities. As an example, the Olympics is an event, and playing the guitar is an activity.

An event consumes neither time nor resources. Since it is only a point in time, no duration, manpower, or cost is associated with an event.

The event does not take up any time or resources.

What is an Event? In Simple Words for Students

An event is a point in time that shows something has started or something has finished. An event is not a job. An event is only a checkpoint. It takes no time and no resources.

Simple Definition: An event is a point in time that marks the start or completion of one or more activities.

Real-Life Example 1: Building a House

Suppose you are building a house. The work is done in this order:

Dig the foundation.
Pour concrete.
Build walls.
Construct the roof.

Now identify the activities and events:

Activity (Work)	Event (Checkpoint)
Dig foundation	Foundation digging completed.
Pour concrete	Concrete completed.
Build walls	Walls completed.
Construct roof	Roof completed.

Here:

Digging = Activity
Foundation completed = Event

Notice that "Foundation completed" is not work. It simply tells us that the work has finished.

Real-Life Example 2: Cooking Tea

Imagine you are making tea.

Activities are:

Boil water
Add tea leaves
Add milk
Serve tea

Events are:

Water boiled
Tea leaves added
Tea ready
Tea served

Again,

Boiling water is work (Activity).
Water boiled is an Event because it is just a point indicating completion.

Real-Life Example 3: School Examination

Activities:

Fill examination form
Study
Write exam
Evaluation

Events:

Form submitted
Exam started
Exam completed
Result declared

Here, "Result declared" is not work. It is simply an event.

Why Does an Event Take No Time?

Imagine looking at a clock. At exactly 10:00 AM, the teacher says, "Exam Starts."

Does saying "Exam Starts" take one hour?
No.
It happens instantly. That instant is an event.

After that, students write the paper for 3 hours. Writing the paper is the activity.

Representation of an Event

In a network diagram, an event is usually represented by a circle (node) with an event number written inside it.

Activities are represented by arrows connecting two events.

Here, A is known as the activity, as shown follows:

If an activity starts at event i and ends at event j, it is denoted as:

Activity (i, j)

where:

i = Tail Event (Starting Event)
j = Head Event (Ending Event)

In other words, activity (i-j) always denotes a job that begins at the event ( i ) and ends at the event ( j ), as shown follows:

Rules for Numbering Events —

The following rules are generally followed while drawing network diagrams:

The direction of movement is from left to right.
The number for the head event is greater than the number for the tail event. (Fulkerson's Rule).

Characteristics of an Event

The important characteristics of an event are:

An event represents the beginning or completion of one or more activities.
It is a point in time and has no duration.
It does not consume time or resources.
It is represented by a numbered circle (node).
An event cannot occur until all preceding activities leading to it have been completed.

Types of Events in a Network Diagram

We know that an event denotes the completion of one activity and the beginning of another. Based on the flow of activities, events are classified into three types or categories:

1. Merge events,
2. Burst events, and
3. Merge & Burst events.

(1) Merge Event: A merge event occurs when two or more activities come from an event.

A merge event is an event into which two or more activities enter. The event occurs only after all incoming activities have been completed.

(2) Burst Event: A burst event occurs when more than one activity leaves an event.

A burst event is an event from which two or more activities originate. After this event occurs, multiple activities can begin simultaneously.

(3) Merge & Burst Event: At the same time, an activity can be merged and burst.

An event may act as both a merge event and a burst event at the same time. In such a case, two or more activities enter the event, and two or more activities also leave the same event.

Basic Axioms of Network Representation

The following two axioms underpin network representation:

(a) An event is not considered complete until all of the activities leading up to it have been completed.

(b) No further activities can begin until the tail event has been reached or completed.

These principles ensure that activities are performed in the correct sequence and that project dependencies are accurately represented in PERT and CPM network diagrams.

Common Mistakes About Events in Network Diagrams

Students often confuse the concept of an event with an activity. The following are some common mistakes and their correct explanations.

Common Mistake	Correct Explanation
Incorrect: An event is a task or work.	An event is only a point in time that indicates the beginning or completion of one or more activities.
Incorrect: An event consumes time.	An event has zero duration and does not consume any time.
Incorrect: An event requires resources.	Events do not consume manpower, materials, equipment, or cost. Only activities require resources.
Incorrect: A merge event is where activities leave an event.	In a merge event, two or more activities enter the event.
Incorrect: A burst event is where activities enter an event.	In a burst event, two or more activities leave the event.
Incorrect: An activity can start before its tail event occurs.	An activity can begin only after its tail event has occurred.
Incorrect: An event is complete when any one preceding activity is completed.	An event is completed only after all preceding activities leading to it have been completed.
Incorrect: Event numbers can be assigned randomly.	The head event number must always be greater than the tail event number to maintain a valid network structure.

Exam Tip

Remember the following points for examinations:

Event = Point in Time
Activity = Work or Task
Event = Zero Time + Zero Resources
Activity = Consumes Time and Resources
Merge Event = Activities Enter
Burst Event = Activities Leave
Head Event Number > Tail Event Number

Simple Tricks for Students to Learn

Activity vs Event

Imagine this simple sequence:

(Event 1)
●
\
Fill Admission Form
\
●
(Event 2)

Event 1: Admission process starts.
Activity: Fill the admission form.
Event 2: Admission form submitted.

The work happens between two events.

Easy Trick to Remember

Whenever you see words like:

Started
Completed
Approved
Received
Submitted
Finished
Declared
Installed

👉 These usually represent Events.

Whenever you see words like:

Build
Write
Prepare
Install
Paint
Dig
Cook

👉 These are Activities because they involve doing work.

One-Line Formula

Event → Activity → Event

Example:

Foundation Started
↓
Dig Foundation
↓
Foundation Completed

Here:

Foundation Started = Event
Dig Foundation = Activity
Foundation Completed = Event

Super Easy Memory Trick

Think of a cricket match:

Match starts → Event
Players play the match → Activity
Match ends → Event

So, events are the START and END points, while activities are the WORK done between them.

Once you remember this idea,

"Event = Checkpoint,
Activity = Work"

network diagrams become much easier to understand.

About the Author

Lata Agarwal

Mathematics, Science and Astronomy professional, M.Sc. and M.Phil. in Maths with 10+ years of experience as Assistant Professor and Subject Matter Expert.

Author at Prinsli.com

References

Hamdy A. Taha, Operations Research: An Introduction, 10th Edition, Pearson.
Frederick S. Hillier and Gerald J. Lieberman, Introduction to Operations Research, McGraw-Hill Education.
J. K. Sharma, Operations Research: Theory and Applications, Macmillan India.
Kanti Swarup, P. K. Gupta and Man Mohan, Operations Research, Sultan Chand & Sons.
S. D. Sharma, Operations Research, Kedar Nath Ram Nath.

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