# Dummy Activity in Network Analysis

A Dummy activity is a type of operation in a project network which neither requires any time nor any resource. It is an imaginary activity shown in a project network to identify the dependence among operation.

The representation and more features of dummy activity in network analysis are explained below.

## Features of Dummy Activity

Dummy activity forms a connecting link for control purpose or for maintaining the uniqueness of the activity. A dummy activity is used to maintain the network logic and to avoid ambiguity.

An activity followed by a dummy activity can only be completed once the activity or activities preceding the dummy activity is completed.

A dummy activity can be either critical or non-critical. It becomes a critical activity when the earlier start time and the latest finish time are equal.

## Representation of Dummy Activity

A dummy activity, being an imaginary or redundant activity, is represented by a dotted line with an arrowhead. The numbers on the terminal nodes are used to represent a dummy activity. A dummy activity is clearly explained by an example in figure-1.

Consider 2 sets of activities as Set 1 and Set 2.

Set 1

A: Delivery of New Machines

B: Install New Machines

Set 2

C: Remove Existing Machine

D: Dispose Existing Machine

As shown in the network above, A and B can be performed serially. The activities C and D can also be performed serially. But practically, the activity D of set 2 can only be performed after completing the activity B of set 1.

So a dummy line is drawn between node 2 and node 5 indicating that the activity D can only be performed after the event 2 is over.

## Uses of Dummy Activity

The dummy activity serves the following purposes in a project network:

1. Grammatical Purpose
2. Logical Purpose

### 1.Grammatical Purpose

A dummy activity can be used to prevent two arrows with a common beginning and end points. This can be explained by an example as shown in the figure-2.

Consider the arrows of activities A and B. Both start from node 1 and end at node 2. This arrangement is difficult to conduct computations and the network loses its uniqueness in its identification. Such inconvenience causes frequent mistakes during network analysis.

### 2. Logical Purpose

It is difficult to represent an activity having two sets of operation running parallel to each other in a network. The use of a dummy activity helps to give a logical representation without difficulty in interpretation.

For example, consider the network given in figure-3 where there are two activities Q and R. Both have a common end node. The predecessors of Q are O and P while that of R are P and N.

## Rules for Using Dummy Activity in Network Analysis

Too much use of dummy activities in a network creates confusion. Initially, liberal use of dummy activities is followed to the requirements of inter-relationships between activities. This network is later modified stage-by-stage by removing unnecessary dummies to finally obtain a simple network. This is explained by an example below (Figure-4).