Dummy Activity

• A dummy activity is an imaginary activity
• It is represented by the dashed arrow.
• Does not consume any time and resource.

Example of Dummy Activity

Purpose of Dummy Activity

There are two reasons to used dummy activity one is grammatical purpose another one is the logical purpose.

1. Grammatical Purpose

We will try to understand it by seeing the picture. The activity ‘A’  starts from node 1, and B also starts from node 1, and both activities end at node 2.  

We can represent it as follows:

• A ⇒(1,2)
• B ⇒(1,2)

Now you can see that both identities are the same, so they lose their identity. Here, the main logic is that they both start at the same time and also finish at the same time. To find out their identity with logic, we used a dummy activity as fig.2

Now we can represent like as
A ⇒(1, 3)
B ⇒(1, 2)

2. Logical Purpose

You can not provide the same activity twice in your network diagram.  

In this above picture, it shows that the P activity is two times in a single network diagram. So for avoiding these types of error in the network diagram, we provide dummy activity.

Rules for Removing Redundant Dummy Activity

You can use as many dummy as you wish, but sometimes it asks to remove dummy which is not required. Five golden rules used for removing redundant dummy. 

Rule: 1

If dummy originated from one node.

Here, P1 dummy is not required

Rule: 2

If a dummy is the only arrow going into a node.

Here, A3 dummy is not required

Rule: 3

If two activities have set of a predecessor.

Rule: 4

If two activities have the same set of a successor.

Here, A2 and B2 dummy activities are not required

Rule: 5

Do not repeat already implied logic.