Activity in Network Diagram Analysis: Types and Use of Dummy Activity

Introduction

In Network Analysis, every project is divided into a number of smaller tasks called activities. Any individual operation, which utilizes resources and has a beginning and an end, is called an activity.

In a network diagram, an activity is depicted by an arrow, where the tail represents the start, the head indicates the direction of progress, and the length typically signifies duration.

Activity in Network Diagram Analysis: Types and Use of Dummy Activity

These activities are connected in a logical sequence so that the project can be completed in an organized manner. Before learning CPM and PERT, it is essential to understand the concept of activities and the role they play in a network diagram.

Some activities can begin only after certain other activities are completed, while others may be performed simultaneously. Understanding these relationships helps project managers prepare an accurate network, estimate the project duration, and monitor progress effectively.

What is an Activity in Network Analysis?

An activity is a specific task or operation that forms part of a project. It requires time and may also require resources such as labour, materials, equipment, or money for its completion.

In an Activity-on-Arrow (AOA) network, an activity is represented by an arrow, where the direction of the arrow indicates the flow or progress of the project.

Real-Life Example

Suppose a company is constructing a new office building. Activities such as site preparation, foundation work, brickwork, electrical installation, painting, and interior finishing are all project activities. Each activity must be completed according to the required sequence before the building is ready for use.

Types of Activities in Network Analysis

Based on their relationship with other activities, project activities are commonly classified into the following types:

Predecessor Activity
Successor Activity
Concurrent Activity
Dummy Activity

Note: Based on Sequence and Relationship, activities are classified into 3 main types (Predecessor, Successor, Concurrent), with Dummy Activity used as a logical connector.

Each type is explained below.

1. Predecessor Activity

A predecessor activity is an activity that must be completed before another activity can begin. Without completing the predecessor, the next activity cannot begin.

Activity that must be completed immediately prior to the start of another activity, is called predecessor activity.

Example

In the construction of a house, the foundation work must be completed before the construction of walls can start. Therefore, foundation work is the predecessor activity.

Exam Tip

Remember that a predecessor activity always occurs before the activity that depends on it.

2. Successor Activity

A successor activity is an activity that starts only after one or more predecessor activities have been completed.

In other words, it follows another activity in the project sequence. This activity cannot start until one or more preceding activities are completed. It immediately follows its predecessor.

Activities that cannot be started until one or more other activities are completed but immediately succeed are called successor activities.

Real-Life Example

During the construction of a building, painting can begin only after the plastering work has been completed. Therefore, painting is the successor activity.

Exam Tip

A successor activity cannot start before its predecessor is completed. Always mention this point in examination answers.

3. Concurrent Activity

Concurrent activities are two or more activities that can be performed simultaneously because they do not depend on each other for their execution.

Executing concurrent activities at the same time can reduce the overall project completion time and improve resource utilization.

Concurrent activity is defined as any activity that can be completed at the same time (concurrently). An action can come before or after an event, or it can happen at the same time as one or more of the other activities.

Real-Life Example

While constructing a shopping mall, the electrical wiring and plumbing work in different sections of the building may be carried out simultaneously. Since these activities can proceed independently, they are concurrent activities.

Exam Tip

Do not confuse concurrent activities with predecessor or successor activities. Concurrent activities are performed at the same time, whereas predecessor and successor activities have a sequential relationship.

Operations Research Notes

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

Summary of these 3 Activity Types

Activity Type	Meaning	Simple Example
Predecessor Activity	Must be completed before another activity starts	Foundation before wall construction
Successor Activity	Starts after the predecessor is completed	Painting after plastering
Concurrent Activity	Can be performed simultaneously	Electrical wiring and plumbing in different areas

4. Dummy Activity

What is a Dummy Activity in Network Analysis?

A dummy activity is an activity that does not represent any actual work. It requires no time, no cost, and no resources. Its only purpose is to show the correct precedence (logical dependency) between activities in an Activity-on-Arrow (AOA) network diagram.

Unlike a real activity, a dummy activity is introduced only to maintain the correct logical sequence of a project. It is usually represented by a dotted arrow in a network diagram.

A dummy activity is one that does not consume any resources and merely represents technological dependence. It only indicates precedence relationships and requires no time or effort to use.

A dummy activity is a simulated activity that consumes zero time and zero resources. It is represented in a network diagram using a dashed or dotted arrow (-----›). Its primary purpose is to maintain logic, indicate precedence relationships, and prevent grammatical errors in the network layout.

When to Use a Dummy Activity in Network Analysis

In the following scenarios, a dummy activity is inserted into a network diagram to classify the activity pattern:

To distinguish activities with similar starting and ending points, that is, Resolving Parallel Activity Identifiers.
Identifying and maintaining the proper precedence relationship between activities that are not linked by events.

Scenario 1: Resolving Parallel Activity Identifiers

When two or more activities share the exact same starting event (node) and ending event (node), they run completely parallel. To ensure that each activity maintains a unique identity (defined by its unique start and end node numbers), a dummy activity is inserted.

Example: If Activity A and Activity B both start at Node 1 and end at Node 2, a dummy activity is introduced to route one of the activities through an intermediate node.

Scenario 2: Clarifying Complex Precedence Logic

Dummy activities maintain proper precedence relationships between activities that share some, but not all, dependencies.

Example: Consider a scenario where:

Activity A and Activity B are concurrent activities starting simultaneously.
Activity D depends only on Activity B.
Activity C depends on both Activity A and Activity B.

To establish that C depends on both A and B, without incorrectly implying that D depends on A, a dummy activity is inserted connecting the end of Activity A to the start of Activity C. A dummy activity can be used to deal with such a situation.

When two or more activities would start at the same node (event) and finish at the same node, i.e. they are exactly parallel. Then, between the end of one of the activities and the common finishing node, a dummy would be inserted.

This ensures that each activity has its own description when referred to by its start and finish node numbers. Dummies are frequently used to improve network layout. When they are not strictly required to represent the logic at hand. This is common at the beginning or end of a network, where a number of activities either start from or converge to a specific point.

Real-Life Example

Suppose two construction activities, Foundation Work (A) and Material Procurement (B), can be carried out independently. However, Wall Construction (C) can begin only after both A and B are completed, while Electrical Planning (D) depends only on B.

In this situation, a dummy activity may be introduced in an AOA network to represent the correct dependency without adding any real work or project duration.

Characteristics of a Dummy Activity

A dummy activity has the following characteristics:

It represents no actual work.
It has zero duration.
It requires no labour, material, equipment, or money.
It is used only to maintain the correct logical relationship between activities.
It is represented by a dotted arrow in an Activity-on-Arrow (AOA) network.

Exam Tip

In university examinations, always mention these three points about a dummy activity:

Zero duration
No resources
Used only to show the correct precedence relationship

These points are commonly expected in theory questions.

Key Rules for Using Dummy Activities

Zero Resource & Time: Always assign 0 duration and 0 resource consumption to a dummy activity.

Avoid Redundancy: Do not add unnecessary or redundant dummy activities, as this complicates the critical path calculation.

Visual Representation: Always draw dummy activities using dashed lines with arrowheads (-----›).

Why is a Dummy Activity Used?

A dummy activity is introduced only when it is necessary to represent the correct logical relationship between activities in an Activity-on-Arrow (AOA) network. Since it does not represent any real work, it is used only to improve the accuracy of the network diagram.

The main uses of a dummy activity are explained below.

1. To Show the Correct Precedence Relationship

Sometimes an activity depends on more than one preceding activity, while another activity depends on only one of them. In such situations, a dummy activity is used to represent the correct dependency without changing the actual project logic.

Example

Suppose Activity A and Activity B start at the same time.

Activity C can begin only after both A and B are completed.
Activity D depends only on Activity B.

In an AOA network, a dummy activity is introduced to show these relationships correctly.

2. To Distinguish Parallel Activities

A dummy activity is also used when two activities would otherwise have the same starting event and the same ending event.

Without a dummy activity, both activities would appear identical in the network diagram, making it difficult to identify them separately.

By inserting a dummy activity, each activity receives a unique representation while the logical sequence remains correct.

3. To Remove Ambiguity in a Network Diagram

A well-prepared network should clearly show the relationship among all project activities. If the logical sequence becomes confusing or ambiguous, a dummy activity can be used to make the network easier to understand.

This improves the clarity of the diagram without affecting the project duration or resource requirements.

Remember

A dummy activity is introduced only when required. If the logical relationships can be represented correctly without it, there is no need to add a dummy activity.

In modern project management software, many projects are represented using Activity-on-Node (AON) networks, where dummy activities are generally not required. However, dummy activities remain an important concept in Activity-on-Arrow (AOA) networks and are frequently asked in university examinations.

Exam Tip

If a question asks "Why is a Dummy Activity Used?", write these three points first:

To represent the correct precedence relationship.
To distinguish parallel activities having the same start and end events.
To remove ambiguity and improve the clarity of an Activity-on-Arrow (AOA) network diagram.

Writing these points first and then explaining each with a small diagram usually earns better marks in descriptive examinations.

Difference Between Activity and Dummy Activity

Basis	Activity	Dummy Activity
Meaning	Represents an actual project task or operation.	Represents only a logical relationship between activities.
Time	Requires time for completion.	Has zero duration.
Resources	Requires resources such as labour, materials, equipment, or money.	Requires no resources.
Cost	May involve project cost.	Involves no cost.
Representation	Shown by a solid arrow in an AOA network.	Shown by a dotted arrow in an AOA network.
Purpose	Performs actual work in the project.	Maintains the correct precedence relationship and removes ambiguity.

Common Mistakes Students Make

Students often make the following mistakes while studying activities in Network Analysis:

Assuming that a dummy activity represents actual work.
Writing that a dummy activity consumes time or resources.
Confusing predecessor activities with successor activities.
Thinking that every network diagram must contain dummy activities.
Forgetting that dummy activities are mainly associated with Activity-on-Arrow (AOA) networks.

1. Incorrect: Concurrent activities always start at the same time.

Correct: Concurrent activities can be performed simultaneously because they are independent, but they do not necessarily have to start at exactly the same time.

2. Incorrect: A dummy activity is an actual project activity.

Correct: A dummy activity does not represent any real work. It is introduced only to maintain the correct logical sequence of activities.

3. Incorrect: A dummy activity increases the project completion time.

Correct: Since a dummy activity has zero duration, it does not increase the project completion time.

Avoiding these mistakes will help you write more accurate answers in examinations.

Frequently Asked Questions (FAQs)

Q. What is an activity in Network Analysis?

An activity is a specific task or operation in a project that requires time and may require resources for its completion.

Q. What is a dummy activity?

A dummy activity is a logical activity that has zero duration, requires no resources, and is used only to show the correct precedence relationship in an Activity-on-Arrow (AOA) network.

Q. Why is a dummy activity used?

A dummy activity is used to maintain the correct logical sequence of activities, distinguish parallel activities, and remove ambiguity in an AOA network diagram.

Q. Does a dummy activity consume time or cost?

No. A dummy activity has zero duration, requires no resources, and does not increase the project cost.

Q. Are dummy activities used in every network diagram?

No. Dummy activities are mainly used in Activity-on-Arrow (AOA) networks when they are necessary to represent the correct logical relationships.

Conclusion

Activities are the basic building blocks of every project network. Understanding predecessor, successor, and concurrent activities helps in preparing accurate network diagrams and managing projects efficiently. Among these, the dummy activity is a unique concept because it represents only a logical dependency and does not involve any actual work, time, or resources. A clear understanding of activities and dummy activities is essential for learning CPM, PERT, and other project scheduling techniques in Operations Research.

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.

Tags: use of dummy activity in the network diagram analysis, activity in the network diagram